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  #1  
Old 03-11-2013, 08:22 PM
Chris Hill Chris Hill is offline
 
Join Date: Sep 2010
Location: Del Rio
Posts: 124
Default Safety Discussion #1: Stall/Spin at Low Altitude

I offer this as a part of a weekly series to prompt discussion about specific flight regimes where anyone interested can read and learn from someone else?s misjudgment and the advice and experience of others who participate in the forum.

When you read these accidents and provide input in the thread, please take the perspective that simply not flying or not performing the maneuver is NOT an acceptable solution. Instead, take the perspective that the pilot was definitely going to choose to fly in whatever manner that lead to the crash or accident. In this way, we can provide methods to perform the same maneuver in a safe manner or with risk mitigated through knowledge/prior planning.

That being said, please read the following accident report from the NTSB: NTSB Accident Report

Summary: Two experienced pilots were completing and instructional flight when they stalled and crashed while turning at low altitude in the traffic pattern.

I?ll start the discussion with a look at flight physics. A review of the effect of bank angle on level turn stall speeds?For an RV that stalls at 55 mph in level flight, the stall speed increases by 7% at 30 degrees of bank (Vstall 60 mph), by 40% at 60 degrees of bank (Vstall 80 mph), by 71% at 70 degrees of bank (Vstall 95 mph). If you are slow in the pattern, say 75 mph, and attempt to use a steep bank to correct an overshooting final turn, you are flirting with a dangerous flight regime. Depending on how your RV flies, you may get little to no warning that you are rapidly approaching stall. For example, my RV-8 only gives me about 5mph stall warning with buffet. Banking it up quickly and pulling it around could send me into an accelerated stall before I had time to react. Low to the ground, it would probably be fatal.

I will leave the opening post at that. I hope some instructors will talk about distractions in the cockpit, whether instructing or flying with friends, and how to reduce/prevent those distractions, especially during critical phases of flight such as takeoff/landing.

Hopefully participation here will remind people about this safety topic and keep it ready in the back of their minds when they find themselves in a similar situation.
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  #2  
Old 03-11-2013, 08:49 PM
sailvi767 sailvi767 is offline
 
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Location: Charlotte NC
Posts: 1,388
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The accident report was interesting but its obvious like most NTSB reports for GA accidents little effort went into the report. It looks like they did some phone interviews and a third party inspected the airframe. Not much of a investigation. I suspect however in this case the probably got the result correct.
On the subject of stall accidents keep in mind that stall speed does not actually vary with angle of bank. It varies with G force relative to airspeed which in the end translates to angle of attack. The posted correlations of stall speed to bank angle are based on a level turn which requires increasing G with bank angle to maintain level flight.
This brings into the discussion stall warning. I know there are many posts on the forum that state the RV is so easy to fly that no stall warning is required and a competent pilot should not need it. I believe the opposite. Stall warning is not just a good safety tool its a critical safety tool. A AOA based stall warning with both a visual component and aural alert I personally think should be required equipment on every RV. Its a backup for when things start to go really bad. Regardless of your level of experience things happen. Your on final on a gusty wind day. The aircraft in front of you is flying slower then you thought. You attempt to square your turn to final. At that moment you get a traffic alert from onboard systems or you see a aircraft inbound visually that is not where you expected. Your wrapping up the turn and trying to correlate the traffic that popped up when you get a 10 knot loss of airspeed with the gusty winds. Lots of moving parts to this scenario but a good AOA based stall warning will break the chain of events and perhaps save a life. "Angle Angle Push Push" could be your best friend at that moment!

George
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  #3  
Old 03-11-2013, 09:57 PM
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Toobuilder Toobuilder is offline
 
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Location: Mojave
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Not trying to derail anything here, but just want to point out the fact that all of us learned about the whole bank angle/stall speed/AOA relationship in ground school. ALL of us. We also were shown this effect during flight training, and had to demonstrate accelerated stalls during our checkride. ALL of us.

So of the people here who need a refresher (probably most of us), how many are going to actually going to knock the rust off this very basic maneuver on their next flight, and how many are going to do nothing more than "discuss" the issue in this thread?

...and more importantly, how many of the "discussion only" types will as a result of this discussion add one more line item to their "personal minimums" list restricting the activity and think they are now "safer"?
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Michael Robinson
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Last edited by Toobuilder : 03-11-2013 at 09:59 PM.
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  #4  
Old 03-11-2013, 10:51 PM
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rv6ejguy rv6ejguy is offline
 
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Location: Calgary, Canada
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Quote:
Originally Posted by Toobuilder View Post
Not trying to derail anything here, but just want to point out the fact that all of us learned about the whole bank angle/stall speed/AOA relationship in ground school. ALL of us. We also were shown this effect during flight training, and had to demonstrate accelerated stalls during our checkride. ALL of us.
Absolutely and anyone who is not thinking about this below 500 feet EVERY time they fly should maybe hang up the spurs. This is one of the most important and basic things you learn early on so you don't kill yourself in an airplane. My rule is not less than 75 knots until I am on final and never use more than 45 degrees of bank below 500 feet unless I happen to have over 100 knots on the ASI.

I can't believe how many accidents have involved wracking planes around at low speeds and low altitudes on the turn to final. No excuse for this sort of accident.
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  #5  
Old 03-11-2013, 11:53 PM
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Russ McCutcheon Russ McCutcheon is offline
 
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I have asked passengers including pilots to stop talking/distracting during these critical phases of flight, I also Practice base to final stalls regularly and know exactly what is possible right to the edge and if I where to go past the edge turning final at 500? it would be correctable for me in my airplane, knowing what the limits are and what happens when there exceeded and how to recover the stall by instinct before it develops will save your skin. I practice a lot, all I need is to be left alone to fly the airplane, if someone is attempting to distract me I will shut them right down no problem.

AOA or stall warning devices, I don?t have them, I don?t find them necessary in my airplane but would never discourage anyone from having them.
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  #6  
Old 03-12-2013, 12:28 AM
johnf_1 johnf_1 is offline
 
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Location: santa rosa
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It seems to me that there is the oft repeated "shoulda known better" slogan with the implication the author knows better, and then there is the "equip your way out of the threat" school of thought. Mack Johnston was sitting front seat in the 8 that smacked flat on base to final a couple of years ago at Truckee, and a fine pilot indeed with gobs of 1011 and lesser time. I recall vividly drinking one of his fine beers around a pot belly stove, and he said "Give me the numbers, and I can fly it.". I mention this because I am half the pilot he was, so it can happen to me. I think Russ is on to it: from a human factor POV, it really is where you choose to focus your attention: airspeed seems adequate to me, but it needs watching for absolute and rate, always, when near the ground. Waiting for the bleat of a warning device seems to have given up half the issue: if you are warned, you already have an issue that needs immediate attention, and then you need to perceive and react, properly, and that begs the question: what were you doing before the bleat? And if it caught you by surprise, now that you are in the middle of a nasty change of events, will to now spring to the correct perception and reaction having doped off? I have my doubts that will work out well in some cases, anyway. I have no particular wisdom, but my rule is 1.3 stall until the numbers...that is 54-55 KIAS in my plane, and I usually am 56-57 KIAS at that point anyway (yes, I float for 3-4 seconds, but, ****, these planes land short anyway.....). FYI: I stalled inadvertently once maybe 500-600 agl many years ago while doping off and enjoying the rapture of flying on a gorgeous autumn afternoon on final to our local wy 1 above Martini and Prati...dropped the left wing, I pushed and powered..no trouble, but the inadvertence was alarming....hope I go another 1000 or 2 hours before that happens again! MTCents J
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  #7  
Old 03-12-2013, 12:40 AM
johnf_1 johnf_1 is offline
 
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Location: santa rosa
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About Chris Hills' note: just a point about accelerated stalls: top rudder in my plane makes the plane sit up flat though out of power at the speeds I use (I don't snap over 100 KIAS any more since I want my plane to outlast me). Dunno about yours....I can skid my way into stalled wingover (I like to do these as a snap entry...it is not a full snap since you start with 30-45 degrees of bank, but pretty cool and violent if you like that sort of thing)...a fatal at low altitude unless you go all the way round......but coordinated, pretty benign outcome, and if you have top rudder, the plane just sits there for a second after coming up wings level....if you come up with power pretty quick, just fly away. Of course, if you keep top rudder and back stick in, you keep going over, but I am assuming from this discussion we are talking about safety, and not aerobatics.
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  #8  
Old 03-12-2013, 12:46 AM
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Snowflake Snowflake is offline
 
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Good stuff to think about. I personally fly in statute miles, because the Cessnas and other slow airplanes I started on had them on the outside of the dial, and by chance so does my -6. I have set knots as the default on my D10A in an attempt to wean myself off statute numbers though.

In miles, I have been targeting no lower than 80 until on final, and then slowly decreasing to 70-75 depending on whether I want a wheel landing (faster) or a three-pointer (slower). I've caught myself drifting above and below the magic numbers before, and corrected with power and lowered nose to increase speed, but I've never had any indications of imminent stall.

To be fair, I haven't explored the slow turn to final at altitude in this RV... to see how slow it can go or what happens in that last few seconds before it all goes pear-shaped. The last time I did that was during my checkout in a different -6.

Something to try the next time i'm up solo.
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  #9  
Old 03-12-2013, 12:46 AM
johnf_1 johnf_1 is offline
 
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Yikes! Just read the NTSB report, and it is Mack's collision with terra firma. RIP
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  #10  
Old 03-12-2013, 01:10 AM
johnf_1 johnf_1 is offline
 
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Rob: It seems to me that one of the bugaboos of us experimental guys is the calibration issue: one man's 70 statute IAS maybe another's 60 KIAS maybe another's 52 KIAS...who knows unless we are talking CAS? With the admixture of instruments, pitot/static and frankly wishful thinking to support arguments, the discussions are sometimes not fruitful. On the other hand, we can talk about ratios without getting into the calibration issues. 1.3 according to Kirschner, Langewische and at least my experience is a good place to start....but, you have to know stall dirty to derive. I am about 41-42 KIAS stall dirty, so I can derive 55 KIAS over the numbers. MTCents J
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