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Miserable Quality Loves Company - Part 2 Safety Margins

WA85

Well Known Member
I am hi-jacking Captain Avgas's thread and trying to re-focus the conversation to the topic I think everyone is trying to address - avoiding a catastrophic accident resulting in the death of one of our fellow RV pilots.

Disclaimer - I built a very imperfect RV-8 and I have several of the well known quality issues / errors that I wished I would have learned about when I was building my RV-8 (firewall penetration's with poor fire-proofing, my canopy has 4 cracks in it, I didn't prime the a few parts, my avionics wiring isn't perfect) and I can't claim to be a perfect mechanic either.

However I think what many (Vic, Dan, Walt, Bob, Sam) on this forum are trying achieve, is to point out repetitive quality issues that can lead to a catastrophic outcome.

Instead of trying to define what is acceptable "quality", maybe another way to look at this issue is how does the "defect / quality issue", affect the safety margin and the consequences of the defect / quality issue, which could lead to a catastrophic outcome.

There FAA uses FARs (i.e. 23, 25, 27, 29 etc) to try to encourage aircraft manufacture's to design and build aircraft with known factors of safety and safety margins for safe operation of aircraft.

FAA TSO's are another form of known factors of safety and safety margins for safe operation of avionics.

The venerable FAA AC43-13 is a form of known factors of safety and safety margins for inspection and repair of aircraft and systems.

The DoD has Mil-Hdbk-517 which is used to establish equivalent levels of known factors of safety and safety margins for safe operation of aircraft.

The FAA uses a myriad of processes to establish and maintain safety margins for the inspection, maintenance and repair of "certified aircraft".
A&Ps, IA's, Part 145 R-P are supposed to be trained to follow safe maintenance process and use approved parts and procedures.

Often times we see fellow forum members pointing to these very same requirements as a basis for they think is "acceptable quality".

In the EAB world, very few of these "safety margins / requirements" exist, but that is the nature of beast and some people love the freedom to do that they want.

There are routine examples of "doing what we want" in this forum. Some are brilliant ideas, some are foolish.

Who determines what is brilliant or foolish, this seems to be where the emotional rub seems to be.

Those of use who have careers in aviation safety / airworthiness / engineering see things on this thread that scare us and as professionals, we speak up, often to severe criticism.

Maybe a better way to for the aviation safety / airworthiness / engineering experts on the forum to express their concerns is to explain the loss of "safety margin" and the potential outcomes.

One of the great popcorn topics is installing the GPS / navigator antenna under the engine cowling.

Yes, it will and does work.

However, if you read the fine print for the installation of GPS antenna, it does not approve the installation of he GPS antenna in the operating environment under the engine cowling.

Yes, it will work, but have you looked at the loss of safety margin that you created by doing this?

Did you do environmental testing above and beyond what Garmin has done?

Its likely you copied someone else's installation found on the interweb.

During subsequent real world IFR operations in the NAS, you are now betting your life, and those of others, (especially if you are RNAV only) by your decision to install the GPS antenna in an unapproved location...but it works...until it doesn't and you become and accident.

Why is installing the GPS antenna in an unapproved environment a safety issue? If you think about it, that GPS antenna may be the sole piece of equipment that is providing differential GPS signal to provide the required performance to you navigator / auto-pilot to fly an LPV approach to minimums. It also might be the sole position source for your ADSB out data.

Both are safety critical functions, with a single point of failure at the GPS antenna, that could lead to a catastrophic outcome (erroneous GPS position leads to splat with the ground or another aircraft).

There are host of other topics to debate, but maybe the best way to work through these is issue is for every builder and owner operator to ask themselves if their maintenance, inspection or repair actions reduce the safety margins of their aircraft and how they will possibly affect a catastrophic outcome to themselves and others.

Okay , I am out of popcorn.

As a side note, I have found several issue on my own aircraft that I think lack sufficient safety margin and I am working to resolve them. My firewall penetrations are lousy, I need to fire proof them. My canopy has too may cracks now, its structural integrity is questionable...I have new canopy in the works.
 
Your point is valid - but I would submit the question to the audience about how many times has anyone seen a performance-related issue that may be attributable to a FWF antenna location? It's most likely that the antenna mfr does not recommend a FWF location simply because it was never tested - not because there was any indication of a problem.

I've got 850 hours on my installation (x3 antennae) with no failures and no RAIM warnings - just one data point.

It's only a problem, if it's a problem. Otherwise it's a solution in search of a problem.
 
Great initiative!

Here's my $0.02 - worth exactly what you paid for it.

As someone who hiked down the "I know better than Van" path and had to do a long and expensive walk of shame back to the well-traveled path, I would recommend to every builder to carefully read the plans and the manual, and do it as close as possible to the factory recommendations on their first aircraft, and to listen to the wise advice of the "greybeards" at Van's, on VAF, and in your local chapter.

After you have the experience of doing it like the plans say, and you want to do something truly experimental, do it on your second airplane!
 
Interesting approach thinking about safety margins. I'm still building and one of the considerations is where to put the GPS antennae. I'm trying to decide whether to put one or two under the cowling, or one or two on the aft fuselage next to the canopy slider rail, or one under the cowling and one on the aft fuselage.

One of the problems with 2 seat RV aircraft is their small size compared to certified aircraft, so the placement of antennae is a problem, especially with lots of antennae (GPS, XPDR, COM, ADSB, ELT, Stormscope, etc.). Each has its footprint, and most recommend a minimum spacing from other antennae. Another is aesthetics, although this should be secondary to function.

So how should I make this decision for GPS antennae? Garmin has tested positions on the fuselage (forward of the windshield, aft fuselage) and tested them in temperature and environmental conditions as well as for signal masking for general use. But not on RV aircraft! If you position the antennae too far aft (say on an RV-8) due to canopy travel, is the antenna masked by the proximity to the vertical stabilizer? If you position two of them side by side on the aft fuselage on either side of the canopy slide rail are they too close to each other and therefore give erroneous signals to the navigator?

On the other hand, many many GPS antennae have been tested by owners/builders/pilots under the cowling and reported good results over long periods of time. Except for the environmental aspects of heat (which could be mitigated by blast tubes) this seems like the perfect position. Good view of the sky, minimal masking by structure, with the proviso not to use metallic paint on the cowling. So here's an example of an actual installation on an RV aircraft, extensively tested by a large group of pilots under actual operating conditions, with few to none failures reported due to being mounted in a relatively high heat environment. Is the under cowl heat environment in an RV worse than a top of fuselage mount on a certified aircraft operating in Central America or near the equator in Africa? Don't know.

How much and what kind of testing of GPS antennae under the cowling of RV aircraft is sufficient to provide the same safety margin as Garmin's recommendations? Supposing that Garmin's recommendations have a safety margin of 1.0, what is the safety margin of mounting under the cowling? Or conversely, how much is that safety margin of 1.0 degraded by the proximity to the vertical stabilizer, or having two antenna mounted on the top fuselage but within 12" of each other because further apart means the antenna is mounted on a curved part of the fuselage?

I've limited my comments to GPS antennae since that is a problem I'm wrestling with. But the OP's point is really about how much testing is adequate to ensure an adequate safety margin and is well taken. It's easy for primary structure where the FARs provide lots of guidance. It's not so easy for systems and especially for system to system interactions.
 
That actually makes sense. Focusing on safety margins is exactly what one should do.

E.g. in the GPS example. I fly VFR only and look outside while I do so. So for me a GPS failure is a low risk issue. Rather an inconvenience than a safety issue. So I don't obsess about it (actually put mine in the wing tip as its the only place I get acceptable coverage inverted to pass ADS-B. Under cowling the engine blocks the signal inverted... ). If I flew hard IFR I would think a lot more about it.

Similarly with a screwed up rivet. Might not be up to FAA standard but Alcoa says they get a large fraction of their strength and if it is one in a particular area the reduction in margin is low. So in that case the important question is not is the rivet screwed up or not but where is it and how many others are screwed up too.

If people would comment using that approach you would not only get more adoption but we would learn more from each other too.

This of course requires the commentator to understand the actual problem not just be able to cite some regulation ... . I have the feeling that there are many long term experts around that can't explain why they are doing what they are doing beyond because the FAA says so.

Oliver
 
Great initiative!

Here's my $0.02 - worth exactly what you paid for it.

As someone who hiked down the "I know better than Van" path and had to do a long and expensive walk of shame back to the well-traveled path, I would recommend to every builder to carefully read the plans and the manual, and do it as close as possible to the factory recommendations on their first aircraft, and to listen to the wise advice of the "greybeards" at Van's, on VAF, and in your local chapter.

After you have the experience of doing it like the plans say, and you want to do something truly experimental, do it on your second airplane!

I am adding my 0.02 regarding the Vans FWF plans :D. I think the RV FWFs vary a lot from one airplane to the next is the Vans FWF plan is all over the map. Even when the use of the fuel flow transducer is prevalent with all the newer build, Vans still hasn't update the plans to include it, maybe except for the newer RV14 kit. BTW this is where the builder gets into problem interpreting into things that are not well documented in the Vans plan.
 
By the book GPS location

I can't recall the exact wording but my GPS manufacturer's instruction was something very close to 'An uninterrupted view of the horizon and free from EMI.'

Near as I could see the only spot I could get that was on the tail, so that's where mine went.

The hangar sharks circled and the frenzy began regarding flow disruption and flutter etc. I studied similar shapes, talked to a few engineers, and went ahead keeping the 'E' in experimental.

I am still here after approaching Vne several times splitting the true airspeed difference when testing was at altitude and can report a very happy location there on top of my tail.

So even when one comports to the letter of the manufacturer's instruction there is still room to cite safety margins. Tradeoffs right?
 
Another thing

One another little issue is that while failure of an IFR GPS in VFR conditions is a "nothing" you will lose your ADSB out WAAS position which can be a nuisance if you happen to be sitting on the ground at a big airport. Safety issue...no, it would be a PIA though
 
Vans puts the GPS antenna under the cowling on the 12. Several hundred flying and no problems reported on the forum that I can recall.

Jack
 
OK, I'll play...

The GNS430W Installation Manual says exactly nothing about mounting the antenna under the cowling. Lots of "should" statements about avoiding masking by the airframe, distance from comm antennae, etc. But not a word that would cause an under-cowl mounting position to "violate" one of the *guidelines* (not requirements).
 
GPS antenna operating environments

A few examples of GPS antenna temperature operating limitations;

Garmin's GA 35 / 36 are manufactured and tested to meet TSO575-93G. The temperature operating limits are -67 F to 185 F ( -55 C to + 85 C) with no significant degradation of performance.

Garmin's GA 37 is manufactured and tested to meet TSO2300-126G. The temperature operating limits are -67 F to 185 F ( -55 C to + 85 C) with no significant degradation of performance.

Dynon lists their GPS 2020 antenna operating limitations as -40° F to 140° F (-40° C to 60° C) with no significant degradation of performance.

NOTE, one of the TSO's lists a maximum variation of dB gain with temperature variations.

Several Dynon GPS 2020 antennas have deformed from heat while mounted on the glare shield.

The highest temperature that one might expect with no degrading of performance is 185 deg F.

One might suppose that the inflight FWF temperatures are slightly below 185 deg F. Could one guarantee that the post shutdown FWF environment won't exceed 185 deg F on a hot Texas summer day?

Its all good until it isn't.

Fly Safe
 
Thanks for posting the temperature limits from the TSO documents. I re-read Garmin's GA 35/36/37 Antenna Installation Instructions 190-00848-00 and 1.3.4 Environmental Specifications does list these TSO documents, but refers the 'installing agency' to the 'Dealer's Only' portion of their website to be able to access these documents. I don't have a dealer login.

Section 2.2 details the antenna mounting location. On the 2 seat RVs I'm not sure it is possible to comply will ALL of the guidelines. In particular 1. which states as close to level as possible (violated by aft fuselage mounting), 2. which states farther away from the tail section to avoid airframe shadowing.

So except for the temperature requirement (not explicitly stated in the Antenna Installation Instructions), the under cowl location best satisfies all of the guidelines.

The only other place I can think of that satisfies all of the guidelines for a side by side RV would be on a tray under the canopy just ahead of the baggage bulkhead. Anyone care to post pix of this installation and report on performance?
 
Wow, this drifted off hard into the land of GPS antenna location arguments ... !!

Back to the OP's original discussion point please.

And that cotter pin comment! Tail's for sure gonna fall off! :eek:
 
One might suppose that the inflight FWF temperatures are slightly below 185 deg F. Could one guarantee that the post shutdown FWF environment won't exceed 185 deg F on a hot Texas summer day?

That would be "storage environment" versus "operating environment". Nobody cares if the variance exceeds limits when the unit is powered down - as long as it returns to acceptable limits after cooling again.

And no, I have not measured the under-cowl temperatures after shutdown - but I certainly don't dispute that they get HOT. I open my oil access door on shutdown to let it breathe, that certainly helps.

5 Texas summers so far for me (and just starting the 6th) with 1 Dynon and 2 Garmins under the cowling, 850 hours. So far it appears any variances disappear when temps come back down.
 
A data point on under the cowl temps. This tape records the highest temp reached. Readings on both pMags match.

Carl
p-Mag-temp-6-14-20.jpg
 
Wow, this drifted off hard into the land of GPS antenna location arguments ... !!

Back to the OP's original discussion point please.

And that cotter pin comment! Tail's for sure gonna fall off! :eek:

Clear violation of AC 43.13 I think so we should form a mob and make sure Stan gets grounded instantly!

;)
 
360 View, level, no EMI

First time trying to attach pic, hope it works.
 

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First time trying to attach pic, hope it works.

Keeping to the "safety margins" theme, this installation may at first blush appear to meet the manufacturer's installation recommendations. However, when checking the TSO documents we'll find the antenna has been tested for lightning only when installed in the swept stroke zones of an aircraft. Normally this means somewhere on top of the fuselage itself.

Is there a degradation of safety margins associated with this installation? One could argue that an increased probability of the antenna becoming a lightning attach point represents a degradation of safety margins.

Equally, one could argue that, by placing the antenna in a locale free from other negatively-impacting factors (temperature, obstruction, being level with the horizon, etc) this location actually provides an enhancement in overall safety margin for the majority of the time.

There is no truly black-and-white answer to this question, only varying shades of grey, with each of us applying our own personal 'quality factor' to the variables before we produce our final assessment as to whether this particular shade of grey suits our own personal safety margins. This last point is critical - what's a show stopper to one person is no big deal to another person.

Coming back to earlier comments about GPS antennae mounted in the engine bay, an earlier poster provided temperature ranges for various antennas along with the caveat that operation within these temperatures would cause no significant performance degradation. Given that GPS antennas contain active pre-amplifiers, it's the noise contributed by those pre-amplifiers that's of concern. Generally speaking, the warmer the amp, the noisier it becomes (referred to as gain over temperature or g/t). Simply put, keeping the antenna cool means the GPS receiver is getting a better, more reliable signal. As with all things, you, the builder, must decide how much of the safety margins built into the GPS equipment you are comfortable eroding by intentionally choosing a consistently hot antenna mounting location.

Oh, one other interesting data point... When a G3X-equipped Grumman AA5B Tiger was being shown off at Oshkosh at the Garmin booth I noted the glaringly-white GA35 antenna was mounted on the glareshield. Now how much safety margin was eroded in that installation by placing that glaring white blob of plastic in front of the pilot's eyes? It's all a trade-off where we get to choose the compromises with which we are comfortable.
 
Lightning Swept Zones

My rated navigation antenna is TSO 190C compliant when installed in 'Lightning Zones 1C(tail), 2A or 3' It's not in this pic but the TSO'ed antenna is probably in zone 2A (undetermined as far as I know) in the typical location on the back. The antenna in the pic isn't TSO rated.

Anyone know whether the RV-7 has undergone the FAA lightning zone determination process? My guess was no, so I didn't actually pursue where the available zones are. But aircraft dorsals aren't exclusively examined and rated. They do the whole airplane. Without the actual engineering I would guess my location in the pic is 1A, or maybe 1C if the nonconducting section of the tail fairing was sufficient to justify that. There is a chapter in the attached link that talks about non-conducting fairings. So, 1C (the tail) would be a TSO 190C compliant location if it were so determined, but to my knowledge there are no Swept Zones determined for the RV-7. Interesting reading about the swept zones if anyone is interested: https://www.faa.gov/regulations_policies/rulemaking/committees/documents/media/TAEeeT2-12111992.pdf

Yes, emphatically! The discussion about safety margins is always a discussion of tradeoffs.
 
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