GPS altitude

I've noticed a difference between my altimeter and my GPS altitude. I use a portable Garmin Aera 560 that is not wired to my encoder or anything like that. I never really give it much thought because I only fly VFR. I've been thinking perhaps I should clarify that difference.

I would like to discuss the following scenario in an effort to fully understand what my little box can and can't do.

You are flying VFR over the top. Beautiful up there and only another 40 miles til clear skies. Your trusty engine stops... You troubleshoot as much as you can but you cannot restart. You are sinking into the clouds below. Reported ceilings are less than 500'. You know you will be making an off field landing. You are confident you can hold the aircraft straight and level as you are descending at best glide through the clouds. As if you aren't rattled enough, the GPS starts screaming "OBSTACLE" or "TERRAIN". You were savvy enough to switch to the "Profile" view of your GPS but since the GPS altitude is different than your actual altitude, you're not sure what to trust. How do you decide which way to make corrections to give you the best odds of survival?

(This same discussion could be used if you got caught scud running, I don't want to discuss the decision to go VFR on top or VFR in low weather conditions. I want to discuss options and ways to use the equipment/information that is available should we ever find ourselves in such a dangerous situation)

I know these situations should be avoided at all costs but obviously they DO HAPPEN. I think it's better discussed now than to try and figure it out under duress.

Since most of my flying was in mountain regions, and the fact that I'm a GPS fanatic, I did a lot of experimentation with GPS altitude versus the altimeter. At times, there could be several hundred feet difference. To avoid a solid rock, I'll take the GPS altitude......as long as WAAS is included.

I used XM Satellite weather, which provides altimeter settings as you fly cross country. Their a bit of a compromise between lows & highs, but still a good indication for a current reset of the alimeter. When changing the altimeter to the new setting, I could watch my Garmin six pac altimeter, and the planes altimeter exactly follow each other for a few minutes, then they slowly drift apart. Get a current altimeter setting, and the same happens again. Over & over. I also had a lot of time to compare GPS altitude readouts, to those of various mountain ranges. Given the fact, that barametric pressure is usually on the change, I'll trust the GPS (with WAAS)if needing to make a choice.

L.Adamson

In the situation you mention, I would use a WAAS GPS and Synthetic Vision any day over the altimeter!

TK, Two lines of reasoning come to mind but they lead to similar places. Which altimeter to trust seems to me a pointless exercise.

First, in my experience the altimeter reading from a sensitive altimeter is usually pretty accurate if you have a reasonably close airport to set it to. I pay attention when setting this at my home field and I'd guess a 10-15' error is typical.

The altimeter reading from a GPS seems to vary a lot, both in precision and accuracy. I'm using a Garmin Aera 510 and an iFly 720, both WAAS enabled and I figure I'm usually within 100'.

So, for absolute altitude, I put more trust the sensitive altimeter, but for terrain, I'd be looking at the GPS to lead me to the lowest terrain. The bottom line is knowing your absolute altitude is a lot less important than knowing where the mountain is and where the terrain falls away. The terrain info might save your life. All I can imagine knowing which one is closer to telling the truth will do will let you calculate how long until you'll be on the ground (should I say a short prayer or go for a longer one?).

The second line of reasoning assumes you don't have a decent setting for your altimeter. In this case, don't trust either source of altitude information. There's no need to trust anything - all they're telling you is when they think you'll encounter terra firma. The fact is you'll encounter it when you arrive and since you have no power, you've trimmed for best glide, there's no way to delay that encounter any further. So who cares which one is right (or more accurately, closer - since neither one is likely right)? At this point I'd follow the GPS terrain data to find the lowest terrain until I broke out of the clouds. At that point I'd shift gears and use the remaining altitude to find the most suitable landing spot.

....finally, I hope you get the discussion your seeking and avoid the comments you're seeking to avoid. There's a lot to learn here and I'm looking forward to seeing other opinions.

tkatc said:

I've noticed a difference between my altimeter and my GPS altitude...

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The two devices are measuring entirely different things. The altimeter is displaying a number based on a model of the atmosphere that is unlikely to correspond to the actual atmosphere very often, if ever. The altimeter should show close to actual elevation when it is at ground level at the spot the altimeter setting is for. The GPS uses geometry to calculate altitude. Inherent GPS errors limit the accuracy of that method. The altimeter altitude is likely to get further and further off true altitude the higher one goes. I suspect this is why IFR terrain clearance rules are different in "mountainous" terrain.

tkatc said:

GPS altitude is different than your actual altitude, you're not sure what to trust. How do you decide which way to make corrections to give you the best odds of survival?

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Altimeter altitude is the one to use for air traffic separation and MAY be more accurate at low altitudes right by the airport the altimeter setting is for but in general for any kind of terrain or obstacle clearance I would rely on the GPS altitude

AIM 1-1-19: "GPS altitude should not be relied upon to determine aircraft altitude since the vertical error can be quite large and no integrity is provided.?

Actually, GPS altitude is fairly accurate. True, it is the least accurate of the measurements provided by GPS. However, it is still very accurate in relation to what it actually measures. It measures altitude above the theoretical center of the earth as provided by the algorithm that is used to determine such things. It is not exact, but as it is not influenced by the greatly varying temperature and pressure, it is actually MORE accurate when used for terrain clearance than a standard altimeter (provided an up to date database). It should not be used for maintaining assigned altitude by ATC, as they are using a pressure based system, but should the GPS freak out and say you are about to hit terrain ahead, you would be wise to heed the warning even if your pressure based altimeter is showing you several hundred feet above said obstacle.

Pressure altitude is often not accurate due to several factors, including temperatures that are different than standard temperature and distance from the reporting station (vertically and horizontally), but it tends to be precise enough for our purposes.

This means that by flying by the indicated numbers on your altimeter, everyone may not be flying at the true altitude, but we are all flying at the wrong altitude together.

Hopefully, no one uses TAWS to push the envelope.

A non-aviation GPS accuracy description...

http://gpsinformation.net/main/altitude.htm

GPS altitude is the geoid height above a mathematical ellipsoid that approximates the earth's surface but usually does not match the actual sea level elevation.

aerovin said:

GPS altitude is the geoid height above a mathematical ellipsoid that approximates the earth's surface but usually does not match the actual sea level elevation.

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This. The earth is not perfectly round, it's oblong. However the GPS system has to use a perfect sphere for calculating GPS altitude, also known as "Height Above Elipsoid."

So you can have MSL, AGL, and now HAE however HAE is just your height above the GPS "center of the earth/sphere."

In some places on the planet it's not that big of a difference, in other places it can be a couple hundred feet. It's a big mission planning factor in GPS guided weapons.

As far as synthetic vision/HAE/MSL/AGL... I believe the terrain data is referenced off of HAE, so you SHOULD be ok using it to avoid cumulous granite, obviously the closer you get the more likely CFIT is. It's a strategic tool, not a tactical one. Where's our resident G3X programmer?

Don said:

At this point I'd follow the GPS terrain data to find the lowest terrain until I broke out of the clouds. At that point I'd shift gears and use the remaining altitude to find the most suitable landing spot.

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This is what I would do for sure.

When I got my first GPS, a Garmin 196, I tested the altitude mode by setting it on a porch railing and checking the altitude------it was roaming around a couple hundred feet up and down just sitting there over a period of 10 minutes or so.

I have no idea if the new GPS units are better than my old 196, but who knows??? Just something to consider.

Don said:

....finally, I hope you get the discussion your seeking and avoid the comments you're seeking to avoid. There's a lot to learn here and I'm looking forward to seeing other opinions.

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Lot to hope for thread drift has a pretty solid history here.

Oh boy, altimetry, my favorite subject!

KTOA said:

[...] GPS altitude is fairly accurate. True, it is the least accurate of the measurements provided by GPS. However, it is still very accurate in relation to what it actually measures. It measures altitude above the theoretical center of the earth as provided by the algorithm that is used to determine such things. It is not exact, but as it is not influenced by the greatly varying temperature and pressure, it is actually MORE accurate when used for terrain clearance than a standard altimeter (provided an up to date database). It should not be used for maintaining assigned altitude by ATC, as they are using a pressure based system, but should the GPS freak out and say you are about to hit terrain ahead, you would be wise to heed the warning even if your pressure based altimeter is showing you several hundred feet above said obstacle.

Pressure altitude is often not accurate due to several factors, including temperatures that are different than standard temperature and distance from the reporting station (vertically and horizontally), but it tends to be precise enough for our purposes.

This means that by flying by the indicated numbers on your altimeter, everyone may not be flying at the true altitude, but we are all flying at the wrong altitude together.

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Yes, exactly right! I'll have to steal that quote. Typically the way I explain this is by saying something like: The baro altimeter is an air pressure gauge that bears a reasonable but not exact relationship to your height above sea level. GPS altitude - ignoring for the moment the various possible sources of error - is closer to what you'd measure if you could drop a tape measure out your window and run it all the way down to sea level at your location. That's why terrain awareness systems, both certified and non, typically use GPS altitude to determine height above terrain.

However, this then inevitably leads to questions like "what do we mean by sea level, exactly?"...

Sig600 said:

The earth is not perfectly round, it's oblong. However the GPS system has to use a perfect sphere for calculating GPS altitude, also known as "Height Above Elipsoid."

So you can have MSL, AGL, and now HAE however HAE is just your height above the GPS "center of the earth/sphere."

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Oops, not quite, although you're pretty close. In essence, the geoid is what the shape of the Earth would be if you scraped off all the mountains and covered the whole planet with water. You can think of it as sort of a squashed-looking sphere with lumps on it - the squashed shape (more correctly, an oblate spheroid) comes from the fact that the Earth is spinning on its axis, and the lumps are due to variations in density inside the planet, all of which affects what we refer to "sea level".

A geodetic mathematical model of the Earth's shape, such as is used by GPS, is referred to as an ellipsoid, and it is also an oblate spheroid (squashed sphere) but without the lumps. Here's a useful image from the web that helps visualize the difference between the geoid and the reference ellipsoid:


If you're interested, the ellipsoid model most typically used is called WGS84. The Wikipedia article about it contains enough information to set your head spinning around its own axis (which as we know will cause it to take the shape of an oblate spheroid...). Here's a visual depiction of the height differences between the WGS84 ellipsoid and the actual measured geoid:


Another important point is that your GPS also knows how to approximate the difference between the geoid and the idealized ellipsoid based on its knowledge of where you are - not perfectly, but the net effect is that the "height above sea level" displayed on your GPS (and used for terrain awareness) is closer to true height above the geoid than the red/blue figure above would suggest.

Anyway, if I was going to rephrase what you said to be more technically accurate and convey what I think you meant, I'd state it as: "The earth is not perfectly round or oblong, it's oblong and lumpy. The GPS system uses a perfect oblong shape for calculating GPS altitude, also known as Height Above Elipsoid. Then it subtracts back out a mathematical approximation of the the effect of the lumpy bits, which results in a figure that fairly closely approximates real MSL altitude."

Or, the Cliff notes version, put simply and ignoring sources of error:

• GPS altitude is best for staying away from the ground and sharp things that stick up from the ground.
• Baro altitude is best for staying away from other airplanes and staying out of trouble with ATC.

- Matt

Just another anecdotal point about HAE vs the Kollsman.

While flying last week on an IFR flight I saw a difference between the pressure altimeter and the WAAS GPS of 348 feet in the lovely rain near Rochester, NY. Raised my eyebrows. Made me start wondering if something was wrong. There was nothing. It was what it was.

EDIT ** Ah very nice Matt! And the summary lines are great.
However, with terms like lumpy bits and sharp things, you must be a Python fan.

Thanks for the post! WGS84 is the standard we use, and I'd always wondered just what it was.

Sig600 said:

As far as synthetic vision/HAE/MSL/AGL... I believe the terrain data is referenced off of HAE, so you SHOULD be ok using it to avoid cumulous granite, obviously the closer you get the more likely CFIT is. It's a strategic tool, not a tactical one. Where's our resident G3X programmer?

Click to expand...

Maybe my sideline hobby is a bit morbid, but mostly, I just wanted to prove the value of terrain base GPS. Whenever any of my cross country trips included areas, which were close to sites of CFIT, I made it a point to go there, and check out the life saving benefits of a good terrain moving map. The GPS warnings worked everytime. Too bad the persons involved didn't have one.

Great discussion, some of which I knew already, some which I did not.

Ok....so lets switch gears a bit. We are scud running with a good engine and we run into trouble, the tops are unknown but thought to be VERY high so climbing through IFR weather is not an option for the VFR only pilot. (Of course there may be instances where I would climb to a safe altitude anyway, declare an emergency and ask for help) Based on this thread, I am better off trusting my GPS altitude and terrain warnings. Personally, I would be inclined to make a 180 degree turn and try to find the better weather from which I just came from. If in mountainous terrain, I would also likely be inclined to climb a bit. In flatlands I probably maintain altitude.

So the 180 is successful...my GPS is warning of terrain (yellow not red), do I dip down slowly to try to get under that ceiling?

I suppose one never knows what they will do until they encounter the situation but I'd rather discuss it now rather than later. On one hand you are facing FAR violations, on the other, well...perhaps death. What do you do?

WingX Pro has a profile view that shows you the terrain elevation in the path ahead of you. You will clearly see if you are heading towards higher terrain. It shows your immediate AGL altitude based on its terrain database and your current GPS altitude.

Finally, it also has an obstacle database that will issue a warning if you are within a couple of miles of a tower or other obstacle that is in your path and higher than your current GPS elevation.

tkatc said:

... How do you decide which way to make corrections to give you the best odds of survival?

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If my GPS position is accurate, I think I'd just use which ever altitude reading is lowest as my que. And, I'd be following the terrain guidance from that fancy glass panel I spent so much money on. At that point, I'm thinking its pretty much seat-of-the-pants anyway. I'd think its more about following the terrain downhill rather than worrying about actual pressure altitude. Hopefully I have a good nav-plan that gives me some "outs".

tkatc said:

...You are flying VFR over the top. Beautiful up there and only another 40 miles til clear skies. Your trusty engine stops... You troubleshoot as much as you can but you cannot restart. You are sinking into the clouds below. Reported ceilings are less than 500'. You know you will be making an off field landing. You are confident you can hold the aircraft straight and level as you are descending at best glide through the clouds. As if you aren't rattled enough, the GPS starts screaming "OBSTACLE" or "TERRAIN". You were savvy enough to switch to the "Profile" view of your GPS but since the GPS altitude is different than your actual altitude, you're not sure what to trust. How do you decide which way to make corrections to give you the best odds of survival?

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I have some time flying through the Canadian Rockies with my G3X, in clear VFR conditions. And by "through", I mean below tops.

I turn off "Betty". She can get annoying. That said, if I ever found myself in low vis I'd talk sweet to her and consider her my best friend. I don't know how accurate she is, but if I can't see she is way more accurate than me. The synthetic vision is good too, but I'd never want to be in the soup depending on it.

Back to the OP's point, if you're going down with no power through a cloud deck, your altimeter does you no good because though it may tell you how high you are compared to sea level (and with acknowledements of the previously mentioned limitations of this), it doesn't tell you where the ground is. Being at 4000' is lovely in some places. In other areas it is in the middle of a mountain.

Reality is that there is no one answer for this situation. It is unlikely to happen, but if it does you fly the plane FIRST. Use your GPS, altimeter and charts, help from flight control, be slow and at minimum descent rate, use your checklist (maybe a tank change makes this nothing more than an underwear change?) and above all keep your head. When you break out, pick your spot, and then fly it there. Keep good emerg equipment with you. Carry a locator beacon.

My philosophy and mantra in these situations is that the plane is a writeoff when the engine quits. All that matters at that point is saving the hide of myself and my passenger. Do what you need to do with this in mind.

Well... that was kind of depressing! Thankfully, we can make our planes very reliable!

Most of it has been said. The altimeter is a pressure gauge that just happens to be calibrated in feet. Under most conditions it will give a reasonably accurate approximation depending on the sub-scale settings. It works because everyone uses the same kit. In reality, it may as well be calibrated in "turtles"........

Modern GPS is pretty accurate. I fly B747s and we are getting some kit which gives a good vertical awareness profile using GPS with the rider that it is not intended for navigation but rather awareness. Having said that, if you have an engine failure with an overcast then your best option is probably to follow your GPS terrain information and head for the valley.......

g3xpert said:

Or, the Cliff notes version, put simply and ignoring sources of error:


GPS altitude is best for staying away from the ground and sharp things that stick up from the ground.
Baro altitude is best for staying away from other airplanes and staying out of trouble with ATC.

- Matt

Click to expand...

I love your Cliff notes version. Gives me what I need to know when I am out flying.

tkatc said:

Great discussion, some of which I knew already, some which I did not.

Ok....so lets switch gears a bit. We are scud running with a good engine and we run into trouble, the tops are unknown but thought to be VERY high so climbing through IFR weather is not an option for the VFR only pilot. (Of course there may be instances where I would climb to a safe altitude anyway, declare an emergency and ask for help) Based on this thread, I am better off trusting my GPS altitude and terrain warnings. Personally, I would be inclined to make a 180 degree turn and try to find the better weather from which I just came from. If in mountainous terrain, I would also likely be inclined to climb a bit. In flatlands I probably maintain altitude.

So the 180 is successful...my GPS is warning of terrain (yellow not red), do I dip down slowly to try to get under that ceiling?
I suppose one never knows what they will do until they encounter the situation but I'd rather discuss it now rather than later. On one hand you are facing FAR violations, on the other, well...perhaps death. What do you do?

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I do a lot of mountain ridge crossings wtih a 530W that has the terrain map in it. I watch it for funzies every now and then, and going through the passes I've seen terrain co-altitude that was yellow more than a few times.

tkatc said:

... I suppose one never knows what they will do until they encounter the situation but I'd rather discuss it now rather than later. On one hand you are facing FAR violations, on the other, well...perhaps death. What do you do?

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I keep thinking about this thread so I'll take a stab at it:

Completely agree that working out answers for different what-if scenarios is a good process. But, for this what-if, I would look for root causes rather than a mitigation plan.

I would say the easiest answer applies in this case: prevention. I have little sympathy for someone caught IMC in the mountains. And if you have kids, family, or passengers in the aircraft -- now I'm angry.

My rule is everyone goes home for dinner tonight. The suggested scenarios would have many variables and any potential mitigation effort is not a good choice. You are facing a high probability of death. An FAR violation would not even make my list of concerns.

The CCC: climb, communicate, confess. Get away from the rocks. Use the autopilot, if available. The 180 turn can be a viable option. Keep the turn level. Only climb/descend with wings level. Get more under-the-hood training and try to keep some kind of proficiency. Find the moisture and stay way, way afar from it.

KTOA said:

I would say the easiest answer applies in this case: prevention. I have little sympathy for someone caught IMC in the mountains. And if you have kids, family, or passengers in the aircraft -- now I'm angry.

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You never know for sure. Ten years or so back, a local pilot cancelled his IFR, to land at his local untowered airport. City lights were below. Due to complete darkness, he ended up in a low cloud, became disoriented, only to turn towards the quickly rising mountain. He overflew some foothill subdivisions, before crashing into the terrain.

Actually, I have a lot of real life CFIT stories from around here. VFR one moment, and unplanned, unexpected IMC the next. This includes whiteouts during daytime conditions.

L.Adamson said:

... VFR one moment, and unplanned, unexpected IMC the next. This includes whiteouts during daytime conditions.

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Nope, I quietly call BS. Clouds do not spontaneously appear as if by magic, especially over any significant area of landmass. Temperature & dew point are coming together, which is part of pre-flight planning. Yes, I know it can happen quickly and this needs to be planned for. We see something similar all the time on the west coast when the fog/stratus moves in.

I have zero knowledge of your IFR example -- but I?ll guess you would counter with there was no wx reporting at the airport, and I will counter unused local knowledge, poor instrument skills, poor CRM, poor ADM, poor risk management, poor pre-flight planning, etc., contributed greatly to the accident. And it sounds like he cancelled IFR without the airport in sight, which would be stupid, at night, with nearby mountainous terrain. Just land and cancel on the ground. Tragically the pilot got behind the airplane, probably starting with his wx planning.

I haven?t given it much thought but I can?t think a single CFIT where pilot error was not part of the root cause.

As I previously stated, there are many variables which can be added to tkatc's example: let?s add night, rain, wind, turbulence, equipment failure, rotors, or icing?

And how well trained/proficient is the pilot? Are they painting themselves into a box canyon with their lack of needed skill set? We all make mistakes, sometimes it's the last one.

As each risk factor is added, your personal minimums and go/no go decision should be adjusted accordingly.

This is a good topic to noodle on and a fun discussion.

KTOA said:

Nope, I quietly call BS. Clouds do not spontaneously appear as if by magic, especially over any significant area of landmass. Temperature & dew point are coming together, which is part of pre-flight planning. Yes, I know it can happen quickly and this needs to be planned for. We see something similar all the time on the west coast when the fog/stratus moves in.

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changed & edited the first reply.....

Definitions of whiteouts:

As defined in meteorological terms, white out is when a person becomes engulfed in a uniformly white glow. The glow is a result of being surrounded by blowing snow, dust, sand, mud or water. There are no shadows, no horizon or clouds and all depth-of-field and orientation are lost. A white out situation is severe in that there aren't any visual references. Flying is not recommended in any white out situation. Flat light conditions can lead to a white out environment quite rapidly, and both atmospheric conditions are insidious: they sneak up on you as your visual references slowly begin to disappear. White out has been the cause of several aviation accidents in snow-covered areas.

A snow-covered terrain together with a clouds overcast create a phenomenon called "white-out" that eliminate perception of terrain features (slope) and height above terrain.

I knew five people that have lost their lives due to these conditions. Snow covered terrain & daylight, was an issue in these CFIT situations. You might be surprised of some of the illusions, that you wouldn't expect. A Cessna 310 was flying parallel to the Wasatch front on the east side of Salt Lake City. There were broken clouds below him, and blue sky above. Due to more fuel usage than planned, because of head winds, he decided to make an unscheduled fuel stop. He could see the airport in the distance. With bright snow, haze, and a combination of broken clouds, he had no idea that a lower mountain ridge line was on a 90 degree angle to his flight path. Not until the last few seconds did he see that the terrain was actually rising in front of him, instead of below. He slammed both throttles forward, and yanked the yoke back. It was all he could do. Luckily for him and his passenger, the aircraft hit flat against the 60 degree slope & cartwheeled. They both survived with minor injuries, as the plane sat on this slope about 300' higher than surrounding homes. Once again, a moving map terrain GPS, sure would have been nice.

KTOA said:

I haven?t given it much thought but I can?t think a single CFIT where pilot error was not part of the root cause.

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I've spent 36 years studying CFIT. Call it a side hobby of mine. When a DC-8 slammed into the mountain close to home in 1977, I started thinking about it a lot. There's always a bit of pilot error. If everything worked perfectly to plan, there wouldn't be a problem. It's just bits & pieces that add up. But were talking GPS, moving maps & synthetic vision here. More than not, these devices would have made a tremendous difference to the outcome. I don't care to hear about the perfect flight plan, in which everything "always" falls into place.

L.Adamson said:

<snip> He could see the airport in the distance <snip> he had no idea that a lower mountain ridge line was on a 90 degree angle to his flight path <snip> Not until the last few seconds did he see that the terrain was actually rising in front of him

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The 310 pilot lost positional awareness. Regardless of whether he actually had the airport in sight or not, he did not know where he was, got too low and flew into the ground. The ground did not rise up and smite him.

Straight from the FAA (PHAK Ch. 16):

How To Prevent Landing Errors Due to Optical Illusions. To prevent these illusions and their potentially hazardous consequences, pilots can: … 1. Anticipate the possibility of visual illusions during approaches to unfamiliar airports, particularly at night or in adverse weather conditions. 2. Make frequent reference to the altimeter, especially during all approaches, day and night. … Illusions rank among the most common factors cited as contributing to fatal aviation accidents.

Optical illusions are not black magic voodoo, they exist, there are a lot of them, and we are aware of them. The 310 pilot failed at several levels but the optical illusion only contributed to, but was not the root cause.

L.Adamson said:

Once again, a moving map terrain GPS, sure would have been nice.

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Yes it would have. But he didn't, got behind the airplane, and failed.

L.Adamson said:

I've spent 36 years studying CFIT. Call it a side hobby of mine. When a DC-8 slammed into the mountain close to home in 1977, I started thinking about it a lot. There's always a bit of pilot error. If everything worked perfectly to plan, there wouldn't be a problem. It's just bits & pieces that add up. But were talking GPS, moving maps & synthetic vision here. More than not, these devices would have made a tremendous difference to the outcome. I don't care to hear about the perfect flight plan, in which everything "always" falls into place.

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Disclaimer ? I sound like the grim reaper below but I?m just a happy pilot, who?s enjoyed decades of flying and thousands of safe hours (sometimes due to blind luck.)

Global Comments:

Let?s remember, a pilot always flies into the IMC condition, intentionally or unintentionally. Every. Single. Time.

Whether it?s takeoff, climb, cruise, descent, or approach, the pilot has made the decision to fly. Actions have consequences, and, at times, they are tragic. I keep an open mind but I?ve never read an accident report stating IMC conditions chased down and enveloped the aircraft.

Part of the accident chain is human factors. There is a psychology of awareness of fault, oversight, or omission that pilots either are unaware of or just choose to ignore. Some pilots had no idea they were on a path of destruction until the very instant it occurred.

With respect to the OP asking about getting out of IMC in the mountains: having flown with GPS, SV, HITS, WAAS, TAWS, GPWS, FLIR, etc., they are all absolutely awesome and a paradigm shift. But it won?t help most VFR pilots get out of IMC conditions. Unless they have a good A/P and a cool head, someone still has to fly the airplane and the pretty picture provided by the GPS doesn?t help with that. It?s just a glorified AI. The VFR into IMC statistics coldly bear this out in black and white.

The FARs state explicitly: Each pilot in command shall, before beginning a flight, become familiar with all available information concerning that flight. This includes many different things, including awareness of wx, optical illusions, risk management, and good planning. Maybe a VFR pilot/flight, over mountainous terrain, with sketchy weather, is not a good risk or idea. The old clich? that the accident investigation occurs in CAVU conditions eight hours after the accident has a lot of validity.

This is quickly becoming an outline for a white paper so I?m now bowing out.

KTOA said:

With respect to the OP asking about getting out of IMC in the mountains: having flown with GPS, SV, HITS, WAAS, TAWS, GPWS, FLIR, etc., they are all absolutely awesome and a paradigm shift. But it won?t help most VFR pilots get out of IMC conditions. Unless they have a good A/P and a cool head, someone still has to fly the airplane and the pretty picture provided by the GPS doesn?t help with that. It?s just a glorified AI. The VFR into IMC statistics coldly bear this out in black and white.

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Right off the top of my head, I have a file on two CFITs that involved CFI's. They didn't have color moving map GPS with terrain data bases either. Naturally, since these areas weren't too far way, I visited the sites with my own GPS setup to verify & document. Unless they were just asleep, I can verify that a collusion with terrain, wouldn't have happened, had they had.....

In other words, I had plenty of warning, far, far in advance.

It's another tool. Doesn't warrant flying into IMC if it isn't warranted (plane, conditions and pilot qualified for it), but it is a tool that will help.

Think of the other end. Vfr pilot into IMC with nothing but an altimeter and airspeed, maybe a T&B that works... I know what I'd pick. Just don't let it get you into something you can't handle out of a stupid sense of security.

java said:

It's another tool. Doesn't warrant flying into IMC if it isn't warranted (plane, conditions and pilot qualified for it), but it is a tool that will help.

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Quite a number of CFIT accidents, that I'm aware of, is just a case of very dark nights, in mountainous areas. There was no intention of even flying in IMC.

I have been flying with Skyview for quite some time and while normally I fly in relatively flat terrain (WI and FL) I have taken the plane back and forth across the Appalachians many, many times. Yesterday on my flight across the Apps. I noticed that I was not seeing terrain warning colors (yellow set to appear 300feet below me and red 100ft). I have noticed in the past that they were very accurate and the warning colors appeared when I expected them. This time they did not. In fact, with the Red warning color set to appear at 100 feet below my aircraft, I descended to about 4950 and did not see any colors (600 feet below terrain). The pic below, I had climbed to 5350 ft while next to a 5580ft peak (as depicted on the sectional) and was not seeing any Red or Yellow terrain indications. I was perplexed and surprised. I can only assume that my indicated altitude (baro set auto by Skyview and IFR certified system) was significantly off by GPS altitude? This was VERY alarming to me. I did not check my Indicated GPS altitude. I wish I would have. Next time I will.

It should not be alarming. As has been discussed many times, barometric altitude is only accurate in a 'standard' atmosphere. When hotter or colder than standard you should expect gps (essentially true) altitude to disagree with barometric altitude.
There's a reason IFR rules require a 2000' terrain clearance in the mountains.

BobTurner said:

It should not be alarming...

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Agree, should not be alarming...why set up terrain alarms if they are not actually alarming you of the terrain.

I should also note, that based on visual observation I was below terrain elevation I.E. splat if not paying attention. Also, I re-set my terrain alerts to show yellow at 1200feet below aircraft and just then got the yellow to show up on the terrain map.

My question for Dynon is maybe I am misunderstanding Skyview. Am I not correct that terrain alerting is based on GPS altitude and the alert should go off regardless of indicated altitude. So then why was I below terrain with terrain warning set for 100ft below my aircraft and was not given the alert?

Sorry, I misunderstood. I thought you said you didn't know your true altitude.
If the gps altitude was below the peaks, the map alarm should be red.

apkp777 said:

My question for Dynon is maybe I am misunderstanding Skyview. Am I not correct that terrain alerting is based on GPS altitude and the alert should go off regardless of indicated altitude. So then why was I below terrain with terrain warning set for 100ft below my aircraft and was not given the alert?

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VERY good question.

Did you by any chance take a look at your GPS signal strength and how many satellites you were seeing. I suspect you were in a window of reduced coverage. Altitude is the first thing to degrade on a GPS.

sailvi767 said:

Did you by any chance take a look at your GPS signal strength and how many satellites you were seeing. I suspect you were in a window of reduced coverage. Altitude is the first thing to degrade on a GPS.

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That is what Dynon suggested when I called them earlier today. I didn't think to check the GPS status. Next time.

Mount Salak Sukhoi Superjet 100 (SSJ-100) crash

You have to wonder if faulty GPS reception or processing was a contributing factor. I doubt anyone with synthetic vision would knowing fly into a mountain, and yet they did just that. It only has to be off a little to be deadly. There is a lot of comfort in a few extra thousand feet of clearance altitude.

Why is the accuracy of synthetic vision important?

Ask Nick!

My experience has basically convinced me that I can not trust Synthetic vision. Unless I find there was a fault in my system, I am turning all the terrain proximity warnings off. I am a firm believer in "no information is better than bad information". I have not used Synthetic vision to keep clear of mountains, but it is nice when you scan ahead and see that there is a mountain looming ahead and SV agrees with you.

Tony,

Check your settings. This screen shot was from yesterday's flight:

N941WR said:

Tony, Check your settings.

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Bill, I have my "Red" set to 100ft below and my "Yellow" set to 1000ft.

You were at 2500 MSL and 2517 Absolute. The hills out in front of you range from about 2000ft at the floor and then upwards to about 3500ish and up. Based on your heading and position your course would take you towards peaks of about 3400msl. That compares to my flight scenario I described previously where I did not get terrain indications.

Your OAT was 88F. Or about 20F above standard. I would think if there was going to be a GPS error compared to IALT you would see more than the 17ft that you experienced.

Yesterday while flying in FL, I was at 2500msl and my GPS altitude was 2700 or so. Probably the same OAT and same Longitude. Makes me wonder if my Skyview GPS is not getting accurate ALT info. I will cross check with other GPS altitude indicators on my next flight.

Thanks for taking the time to post your pic!

Tony - as a follow-on to your last post I'll toss in a word here on GPS Altitude.

Altitude is not a separate piece of data broadcast by the satellites but rather it, like your lat/long position, is calculated based on the time of arrival of broadcasts from multiple satellites. As such, if your GPS receiver is tracking a sufficient number of satellites which are in a sufficiently-diverse set of positions in the sky, you will get reasonably good altitude. Of the four dimensions in which GPS provides us a fix, altitude is invariably the least accurate of the bunch.

All of this is to say that if you're getting solid lat/long position information from your GPS you are likewise also likely getting solid altitude. It would be a fairly rare occurrence to not get at least useable altitude data, if not very good altitude data. You can use the on-line GPS prediction tools to determine how good or bad your fix might be at any location and date/time in order to rule out any anomalous conditions which might exist for your test conditions.

For what it's worth, GPS first computes x, y, z and time in Cartesian coordinates in the Earth-Centered, Earth-Fixed (ECEF) reference frame. It then converts that to whatever coordinate system you want it displayed as (subject to whatever the manufacturer has made available)...lat/lon, UTM northings/eastings, etc....referenced to whatever datum plane is selected (NAD27, WGS84, etc.).

That's oversimplifying it a huge amount (e.g., the reference ellipsoid model of the earth, etc.), but just wanted to clarify that conversion to lat/lon is done *after* the [x,y,z,t] vector is computed.

Altitude errors are a function of the geometry of the satellites being received/decommutated, and is in relation to the center of the GPS orbital plane(s), referenced to the ellipsoid or geoid model, depending on the receiver and the options selected.