Tolerating an un-airworthy aircraft. Wing drops at stall
 

Australian Flying magazine November-December 2020, published an article by the editor Steve Hitchin where he talks about brushing up his stall recovery skills using a Piper Warrior at Victorian counttry flying school. The article was well written and entertaining until the bit that said "We had deliberately chosen a Piper Warrior with an unrelenting habit of dropping the right wing at the stall. No point in making this easy" wrote the author.

Later in the article his instructor said " This aircraft has a tendency to drop a wing to the right in a stall"

Certification flight tests by the manufacturer's test pilot require the approach to the stall be flown at one knot reduction per second. In other words not a rapid loss of airspeed as might be expected if the nose was initially pulled up too quickly. In addition the maximum wing drop permitted at the point of stall is 15 degrees.

While the article does not record how many degrees of wing drop on that particular Piper Warrior constituted "an unrelenting habit of dropping the right wing at the stall", the description by the author would suggest it was considerably more than 15 degrees.
That being the case, assuming the approach to the stall was correctly flown at one knot per second speed reduction,the amount of reported wing drop would indicate the aircraft was un-airworthy and the maintenance release so endorsed.

Sharp wingdrops at the point of stall could be due to any number of reasons. For example, damage to the wings, pilot faulty technique, icing on the wings or defective or incorrect rigging. The latter is the most likely in this case.
Rogue flying school aircraft with a reputation for a marked wing drop at the point of stall are often not only tolerated by flying school operators but even encouraged to be used for stall training. Perhaps this is what the author implied when he wrote "We had deliberately chosen a Piper Warrior with an unrelenting habit of dropping the right wing in a stall?" Was this defect recorded in the maintenance release? Buckleys..
An unexpected sharp wing drop during hold-off could lead to an accident; particularly if the aircraft is flown by a student pilot.

Makes me think of the C172 "my" flying school used for introducing us to stalling - stalled left. It needed a bit more than full right rudder to stall straight - I was glad I had flown a lot of RC, felt it when lifting it off, and asked the instructor what kind of stalls we were to perform. "What kind of stalls - what do you mean?".
Just traight ahead, I asked. "Would do nicely", he answered, so I knew that he knew, but knew too that he didn't know that I knew..
I managed to perform three almost straight ahead stalls before he noticed the rudder foot. Then we did some without rudder - felt to me as if a church fell on left wing, with a 60 - 70 degrees wing drop. He asked about spinning, but I decided against it, I would rather be sure we could get it ot of spin too.
It was later sold and had a "landing incident" shortly after.
What I would like to know is, is it normal for flying schools to have something bad-mannered for "waking up people"?

I think it is "normal" for cash strapped flying schools to spend minimum $$$ on aircraft maintenance and to leave some inexplicable items like excessive wing drops at stall to " it always has done that". Particularly with aged aircraft with a phone book list of ex owners and a less than clear maintenance history. I am not saying that this is acceptable, but I have experienced this with a few schools, and I think it is more common than it ought to be, at least in Australia.

it's certified under a different and older certification so it's not obliged to meet new requirements. Archers have very benign stall characteristics in comparison to Warriors..but Never, ever spin an Archer but she really fights the stall both power on or power off even in turns

Maybe the student was on the heavier side. I haven’t flown a flight school Cessna, other than a brand new cirrus, that didn’t drop a wing on stall.

On the day, the stall that tries to kill you will not be some genteel church tea party thing, with perfect manners and plenty of notice. It'll probably be at low altitude, in marginal weather while you are craning over your shoulder looking for the threshold. You'll be out of balance, at low power and missing all the cues. And that's when you'll be glad you understood that correcting a wing drop and recovering needs to be instinctive, whichever way the aircraft takes you. If you can't do that, it's probably not the aircraft that's "un-airworthy" but the pilot.

Not that heavy, 73kg - the instructor was heavier

I disagree.
These aircraft still need to meet their original design and certification requirements.
A consistent wing drop is a sign of a crooked or incorrectly repaired or incorrectly rigged aircraft.
That should not be tolerated.
Unfortunately we’re all guilty of being too eager to fly.

The comments section in the log book allow a pilot to write any defects on the flight . Did the article writer consider putting the wing drop in the log book ?
Snagging an airplane allows maintenance the opportunity to correct defects before they become expensive . Checking the rigging takes minutes , looking at the spar box for damage might require some seats to be removed . But it is always better and cheaper to find a problem on the ground than take a problem flying where it will get real expensive.

I wonder how the pilot discovering the fault and not snagging it , might feel if the next pilot came to grief over his inactions to ground the aircraft or at least prevent stall practice until maintenance has a look . I have grounded lots of airplanes . And Ferried wounded birds home to get fixed .One Cessna Caravan I grounded without looking at it when the pilot told me she saw it move when the forklift went past it . Just a small scratch on the elevator, might not have noticed . the small scratch through a ten foot fulcrum did massive damage to the internals . Not visible until the tail cone was removed . That particular plane repair was way cheaper than the insurance premium increases the company would have suffered if we just let it go to the next inspection . Or we might have lost more revenue through fear from a passenger market that is more frightened by death and dismembering than delays .
When in doubt talk to maintenance and write it in the book . Next Pilot might not be as lucky .

I would disagree with your statement - the aircraft can have never been repaired and correctly rigged & still drop a wing.

Care to explain to us how that would aerodynamically happen in a power off straight ahead wings level coordinated stall as per certification requirements?

A wing drop indicates an asymmetry in how the stall propagates from the wing root to the tip.
A generic light SE GA airplane does not experience a full wing stall just a partial wing stall as indicated by aileron effectiveness well into the buffet.

Unless we are flying an elliptical wing planform with rounded wingtips but that’s not what the original poster was asking about.

I had wondered about that when I read the article. But the author did not "suggest it was considerably more than 15 degrees" nor was there an "amount of reported wing drop". Perhaps he would respond to the question in the next issue of the magazine?

The regulation states something like "it must be possible to prevent more than 15 degrees of roll or yaw by the normal use of controls". It refers to the test pilot not an average pilot as other sections of the regulations require. It therefore does not preclude the behaviour that the author described.

Warriors don't normally behave like that so it seems that there is something not right about the airplane anyway so should be rectified, in my opinion.

Yes, my guess too. I have flown a Decathlon which had a couple of degrees of washin instead of nil to a tad of washout which resulted in unacceptable (to me) behaviour at the stall with a sharp, substantial wing drop which I was unable to prevent. It had been a few years since I had flown that particular one and it had behaved normally before. I can only guess as to how it got like that. It didn't take long to fix it - rig it per the service manual.

Some factors also adding to a wing drop are P factor, slipstream and gyroscopic effects at low airspeed....the warrior is ancient so it's not required to meet current FARs. Although I'm not entirely sure about that, to be absolutely sure one would have to check the FARs that are applicable to the airplane in question. I mean the Warrior is ancient.

Really? I would've thought they'd be almost the same.

They better write the Boeing 717 up as un-airworthy then! :E

Aircraft as mentioned have rigging tolerances of the control surfaces - typically +/- 2 degrees. Unlike other types there is no max between the L/H and R/H, so you can be correctly rigged with a 4 degree difference with the ailerons. There can also be the difference in cable tensions between L/H and R/H, this will have a lesser effect but still an effect.

Then you add in the flaps with a tolerance in L/H and R/H rigging - often streamlined to the aileron so adding to the aileron bias.

The nosewheel can act like a rudder, many owners elect to remove the wheel boots which would normally provide an aerodynamic restoring force to align the nose wheel with the slipstream.

I though the PA28 was a 1950s design and certification basis, CAR 3 ?

Yes, the stall requirements in CAR 3 at the amendment status per the Warrior's (I'm guessing the particular model) Type Certificate Data Sheet are pretty much as I stated in my post #12.

There was allegedly a flying school out there somewhere in our big world, that had one of these aircraft that “violently” dropped a wing at the stall and the aircraft was labelled by the boss as not to be used for student stalls and incipient spin entry, however one day, that aircraft was used for such a lesson and one of the flight crew was fatally injured from a spin that wasn’t able to be recovered from.

Be careful out there, I cringe when I hear stories like the one above!

Huh? Please elaborate. I’ve flown both, and I thought them to be the same airframe, just O-320 engine in the Warrior and O-360 in the Archer, a 20-hp difference. I don’t know them to be different, but if they are, I would expect the Archer to be a bit busier at power-on stalls, but no different with power off. I certainly prefer their tapered wing to the earlier Cherokee wing (commonly called a Hershey bar wing, its shape resembling the chocolate bar, and with similar drag characteristics).

Warriors and Archers have exactly the same stalling characteristics, or at least they should have. All the ones I flew did at least.

Same airframe just 20 hp difference, slightly heavier engine in the Archer which also has a higher MCTOW though I think some later Warriors have the same heavier MCTOW.

With fully castering nose wheels the wheel pants cause the assembly to align with the slipstream.
Like so:


https://cimg1.ibsrv.net/gimg/pprune....4b89849fb.jpeg

With a steering nose wheel there is a lot more friction due to rods and springs and connectors.
The nose wheel may deflect a little during high power operations against spring tension but at idle it should be aligned especially with rudder pedals neutral.
There is no reason for the nose wheel to cause a wing drop in a power off stall.
Zero.

I would agree with your assessment, but all damaged aircraft are not precisely rigged to meet original certification requirements. The human factor changes things.

Cessnas are much easier to rig by way of wing adjustment than Piper. They have a sort of cam mechanism that can adjust angle of incidence. As I understand it, wing rigging for the Pipers consist only in adjusting the wing flaps.

I once flew as an instructor in a Cherokee 6 that was horribly out of rig because of accident damage. The owner had bought it used and was not sophisticated enough to understand it until we flew as part of a 61.56 flight review. It was impossible to trim the ball center in that airplane.

Correct, Cessna’s you adjust the rear wing bolt that is offset in a bushing, rotating the bushing alters the angle of attack of the wing. Pipers you adjust the flaps to correct it.

Forgot to add, on the initial post, has it had the stall strips added, these were introduced to alleviate some of these issues, read the Service Bulletin below.

http://nctc.tk/PIPER/piper%20disc%20.../SB%20916B.pdf

In all of the stalls I've done in the Warrior included a wing drop... nothing huge...I guess next chance, I will observe carefully but I have gotten the wing drop.

PURPOSE: Piper Aircraft Corporation has determined that additional PA-28-161 Aircraft (Warrior II) and PA-28-181 Aircraft (Archer II) not affected by Service Bulletin 916A, may exhibit an undesirable tendency to roll to the right just before and/or during stall high power and aft center-ofgravity loading conditions. If allowed to continue, this may result in roll attitudes up to 60°-90° of bank.

Just imagine the above description as a go-around from a bounce. Low speed, stall warning light or horn sounding, nose high attitude then WHACK the wing drops at 60-90 degrees angle. Even the Wirraway couldn't beat that..

Just thinking that perhaps in some cases it would be how far into the stall you fly...I go beyond buffeting and I completely ignore the stall warning buzzer... when I stall or teach the stall....I wanna see stall!...

​​​​​​There's an old aerobatic trick called a falling leaf in which you hold the plane into a stall and pick up the wing drop oscillations with rudder.

Piper Warriors, Archers and Dakotas all appear to have benign stall characteristics during training, as the CG is near the forward limit. I would suggest a PA-28-151,161,181 or 236 would behave very differently at MTOW with the CG further aft. In fact, this would apply to most 4 seat training types.

​​​​​​ I've recently ran across a similar problem in the Archer with 3 pax,all large men...I wanted to show them a power on stall and the weight and balance was within the limits...but eventually I decided not to and Instead did an emergency descent.

Just thinking that perhaps in some cases it would be how far into the stall you fly

One needs to be cognizant of the relevant requirements and what might have been done during the Type Certification program. Keep in mind that TC requirements vary over the years so a flick through the historical FARs and AC 23-8 revisions can be illuminating.

For instance, I recall a tale from a flight test short course, years ago, where the FT instructor related a tale from a USAF FT training program: the USAF student wanted to see what would happen should the particular aircraft (a well-known civil cabin class twin) be held into the stall. Now, the particular model, at TC, was only tested to stall onset as we were briefed at the time. The (very experienced) FT instructor, having seen it all before, let the student do the deed - result was the usual buffet, etc., followed by a rapid flick into an inverted spin.

Take away message: sometimes a bit of research is useful and, on occasion, this requires some discussion with the OEM FT folk to find out just what might have been done.

We really need to be a bit conservative out there in operating land .... especially should we not have all the details of the story ... PA, I suspect, is better informed than many. However, the many may, on occasion, allow their lesser knowledge overwhelm their caution.

JT that sounds terrifying... Luckily that has never happened to me but nevertheless I definitely take heed in what you just posted

I find it hard to believe that an entry to an upright stall at 1G in a cabin-class twin would lead to a flick into an inverted spin. I'd believe an aggressive entry into an upright spin where the initial motion may result in it being inverted. (JT, I'm happy to demonstrate either outcome to you in a Pitts.)
I know of the fatal accident at the National Test Pilot School with the Derringer (it seems I need to get to 10 posts before I can provide a link) and somewhere there is a description of its stall behaviour which explains how it came to spin from which it didn't recover.

Dave, I just relate the tale, as told by SR (who was the instructor TP being entertained), in Sydney when the folk came out around the early 90s. It was just a sideline anecdote so there was not much detail given but the basics of the story were that the stall was progressed and that the result was, as I recall the commentary, not unexpected, the aircraft being one which the school used routinely, as I recall. His story was that the original certification only looked at the initial stall indications with a prompt recovery. The student's exercise, however, quite obviously pushed the girl well into the stall regime. I presume that the approach to the stall would have been modest although I don't recall any specific comments on that point.

The point in my raising the tale for the newchums out there is that there be dragons and that part of the solution to the problem is knowledge .... You, of course, are at the other end of the hack, flick, zoom spectrum.

I'm happy to demonstrate either outcome to you in a Pitts.

I was quite happy to continue playing with gentle aeros after Aub ran us both through the aero endorsement down at Geelong so, pass on your kind offer - quite happy to go along for a sedate session of aeros, of course. As a sidenote, JSJ was the source of my "Chuck" nickname after a ride in the Chippie at RVAC in days now very long gone by.

This the one ? https://planecrashmap.com/plane/ca/N8602J/

Yes, that's it. SR described it and his subsequent work when at Avalon some years ago.
Sean Roberts' excellent slideshow is available to download at flighttestsafety dot org/36-workshops/workshops?limit=4&start=16 look for “Taming the Stall/Departure Characteristics of the Derringer D-1 Aircraft”

Some info there is relevant to this thread. Especially the bit about "Adding power at the stall (FAA Training) can cause a departure and violates FAR certification criteria." The FAA has since amended their stall recovery procedure in Chapter 4 of the Airplane Flying Handbook and it seems that CASA is on track too with AC 61-16 and, hopefully, their future revised Flight Instructor Manual.

An interesting thread and one, that with a couple of exceptions shows how largely ignorant of the certification standards for Normal category General Aviation (part 23) aircraft most Pruners are.



There was an emphatic statement made at one post about how certification stalling is conducted (Not You DJP) and the 1 kt/sec entry rate with wings level.



Very nice yes, but:

a) This is for the purpose of determination of stall speeds, from which other speeds are factored.

b) It is conducted within several more specific configurational conditions, such as Weight, CoG, power, trim setting and control deflection tolerances and rigging tolerances for primary and secondary control surfaces and for wing incidence, even control cable tensions.

c) Control inputs to arrest yaw are permitted At the point of stall (pitch attitude unable to be maintained by nose up input or minimum speed indicated) the control input is then held for a short additional period to ensure that speed is accurately assessed.

d) Other tests are conducted associated with stall handling with very different conditions associated for the purpose, different CoG, weights, power, turning flight and entry rates are used for the purposes of demonstrating compliance with this requirement, in order to establish the worst cases, for entry and recovery. These stalls are intended to reveal any potentially dangerous cases, and whilst the 15 degrees still applies, in general, it can be added to the 30 degrees of turn, and other cases allow much larger roll excursions when stalling from a turn in one direction, and rolling through level to the opposite wing down.



Bend a Lot, - No! you claim that normal variations of rigging will (or may) take any aircraft outside of its certified standards, not so, these are considered during testing with such conditions as listed above set to CRITICAL limits, which can be associated with either the entry or recovery phases.



John T – you are correct re SR and that story, two men died as a consequence of this type of demonstration when the schools aircraft spun in, - twins are subject to the same standards for stall speed determination, but NOT subject to the same standards for stall handling and asymmetries.



Hitch may be totally correct regarding his comments, AND the aircraft may well have been compliant and well maintained. But it is possible that it or others like it are not, as some aircraft are maintained to a budget and others to a standard. These two are not necessarily concordant.

The IP was making a good point about maintenance, my post is regarding the certification basis, and configuration and conditions applicable to these standards.



HD

For those folks who don't know the last two posters, I suggest that you heed their counsel well - both are longtime greybeards with a wealth of FT and certification background.

Sean Roberts' excellent slideshow

I'd forgotten Sean's presentation at Avalon - Dave's link is well-worth folks' time in having a look at the files. You might need to gloss over some of the graphs a tad but the basics are pretty clear from an educational perspective.

shows how largely ignorant of the certification standards for Normal category General Aviation (part 23) aircraft most Pruners are.

... and generally in the Industry. Dave and I have moved back into the theory training arena to some extent and, while our efforts represent only a drop in the training bucket, we do try to expose the newchums coming through to things a bit outside the MOS syllabus ... Things haven't changed much - I was actively involved in theory training back in the 70s/80s and the overall lack of knowledge and, worse, misinformation and OWTs, which then acquired the status of Gospel through the uncritical telling and retelling thereof, had to be experienced to be believed .. both then and now.

Chris Burns once said something like that to me. Next thing we were upside down in a non-aerobatic aircraft, then falling out of a barrel roll shortly after and he didn’t know where the throttle was in the aircraft (he was a demonstrating passenger).

Watch this!
Hold my beer!

I have seen Piper aircraft that exhibit consistent wing drop for two reasons both of which are hard to see.
The first one was a Piper Seminole that the front wing attach spar bushing which is a press fit into the forward attach tab and that goes through 2 steel plates in the fuselage had been dislodged . The bushing is not clamped between the plates and can be dislodged from the fitting causing the leading edge to change angle of incidence by a couple degrees even with the main spar and rear fitting bolts correctly installed .You won't see it on less the aircraft is jacked up and you lift up the leading edge .The one my workmate found had cut about a third of the way through the bolt.the bushing can be seen by taking out the sidewalls in the cockpit forward of the seats .

The second cause is some one pushing on the Ailerons to move the aircraft which evan when you can't see the deformation by eye makes a difference that can't be rigged out. The only way to check is to swap in a known serviceable aileron and test fly.
This is specifically mentioned in the maintenance manuals.