Spiral Dive

[pelmet]

You are in a light general aviation aircraft climbing in IMC in trim at the best rate of climb speed at a fairly high altitude while on autopilot. You become distracted with something that fell down between the seats and while looking for it you hear that the noise inside the aircraft has changed significantly.

You look up and see that the autopilot has disconnected and are in a spiral dive. Airspeed is near red line, bank angle is 75 degrees(this is from a real accident).

In detail, what are your exact actions for recovery.

[photofly]

Close the throttle, roll the wings level (has your attitude indicator toppled?) and get ready to exert forward pressure on the yoke as the nose comes up, lest you fly a half loop (since your airspeed is about three times your trim speed) and end up on your back...?

[Creston]

Throttle back,
Push/unload wings, (roll rate quicker with wings unloaded)
Roll wings level,
Ease out of the dive.

[Rookie50]

Those are good. I'd suggest a bit shallower climb too than best rate at a safe altitude assuming terrain not a factor. Better cooling, and less abrupt a wing drop if AP kicks off.

Personally, I keep my hand lightly on the yoke during an IMC climb even on AP --- and will generally hand fly the initial climb until safe alt. . Feet stay on the pedals in IMC, too. Just don't trust AP's THAT much.

[donnybrook]

Shouldn't this be in the training forum or something?

[pelmet]

Shouldn't this be in the training forum or something?

True, it should be. But I read two articles about accidents(both PC-12) to aircraft where this was critical, so I thought I would put it in this forum as a subject that perhaps resulted in avoiding a thread at a future date with this type of event....

http://www.aopa.org/News-and-Video/All- ... t/landmark

"In a well-developed spiral, the aircraft will quickly accelerate to well above trim airspeed. In this case, just more than 30 seconds elapsed from a climbing shallow bank to airframe failure. The pilot did extend the landing gear, which is good, but it doesn’t appear that he made a power reduction.

As soon as the aircraft is rolled to level—assuming the pilot gets that far into recovery—it will seek trim speed. If cruise (and trim) airspeed is 150 knots and the pilot manages to get wings level at around 210 knots (and the maneuvering speed is 130 knots), there will be a pitch up as the aircraft seeks to regain trim speed. The pilot must push forward—firmly—to unload the aircraft structure.

Being so far above maneuvering speed, and possibly above redline, it’s not surprising that many in-flight breakups follow a spiral. In some cases a trainee is told that since the aircraft is going down, they need to pull up. In a fully developed spiral, that’s exactly the wrong guidance! It’s going to be wings level and a push"

I can't access the other article yet but I will in the future if able.

[Rookie50]

Stupid question -- perhaps --- will speed build up much faster in the thinner air at FL 250?

[photofly]

Close the throttle, roll the wings level (has your attitude indicator toppled?) and get ready to exert forward pressure on the yoke as the nose comes up, lest you fly a half loop (since your airspeed is about three times your trim speed) and end up on your back...?

Do I get a pat on the back? It seems there must be some benefit in being interested in aerodynamics after all. Fancy that.

By the way - another benefit of closing the throttle in a spiral dive is that it increases your trim speed, which is a very good idea in that situation.

[iflyforpie]

I seriously doubt any light aircraft at any survivable airspeed could perform a half loop with zero power and cruise trim. It would go into a very high amplitude phlugoid oscillation.

[photofly]

I seriously doubt any light aircraft at any survivable airspeed could perform a half loop with zero power and cruise trim. It would go into a very high amplitude phlugoid oscillation.

The aircraft was trimmed for a best rate climb, not for cruise, as posed. But neither outcome is good when you're in IMC. So if you don't want to find out, remember to push on the yoke.

[trey kule]

While this is an interesting discussion, the takeaway on the accidents (if we are discussing the same ones) might have been not a lack of spiral recognition and recovery, but a more basic inability to fly on instruments, and thus trying to reengage the autopilot rather than.......aviate.

years ago I was on a course with a guy who had about 2000 hours in a nice little twin. He was a wizard in AP use. He failed the course because he literally could not a the plane on instruments.

When I read the accident reports, I got the same impression. ( again, if the same accidents).

Basic flight skills deteriorate with the use of modern cockpit automation, and if the skills were never soundly embedded to start with because a pilot went from very low experience to high automation, the result when something goes sideways is often tragic.


As an aside, typically when an autopilot fails it is because something is out of whack..fuel imbalance, trim issue, high angle of attack and unable to hold climb speed..the list goes on, but when it does give up, there are other problems that be a real bugger to identify quickly.
If for example the aircraft is on autopilot and set for a 1000 fpm min climb, the autopilot will trim nose up up up to try and maintain that rate. If it kicks off the trim will not be at climb rate.
Most simple autopilot have only ROC as programmable..also certain pitot static interruptions will cause some simpler autopilots to do the wrong thing, and again, the trim will not be where you think it should be.
Anyway, back to the discusdion at hand.

[photofly]

This fatal spiral dive occurred in VMC, when the occupants of the 172 fell asleep.

http://www.tsb.gc.ca/eng/rapports-repor ... 8o0233.asp

The aeroplane struck the ground on an easterly heading, approximately 45º right bank, pitch angle 7º to 8º nose down, airspeed 140 knots.

[airway]

Being so far above maneuvering speed, and possibly above redline, it’s not surprising that many in-flight breakups follow a spiral. In some cases a trainee is told that since the aircraft is going down, they need to pull up. In a fully developed spiral, that’s exactly the wrong guidance! It’s going to be wings level and a push"[/u][/b]
.

While it might help the student understand the aerodynamics of a spiral dive during training, if you unexpectantly find yourself in a spiral dive there is no need to analyze whether you are going to have to push or pull to recover.

Just follow the recovery procedure that all of us learned in our first 10 hours of flying.

1. Power off
2. Level the wings using co-ordinated aileron and rudder.
3. Recover pitch to a slight nose up attitude. This may require a push or a pull.

A few notes:

If you are in VMC look outside to determine your bank and pitch.
If you are in IMC use your Attitude Indicator to determine your pitch and bank initially. Confirm with your other instruments that your Attitude Indicator has not toppled.
1. and 2. can be done at the same time.
3. can be started when the bank angle is 30 degrees or less, or ground contact is imminent.

[photofly]

It's amazing how pilots manage to make "you're absolutely correct" sound like "you're a complete idiot who has no idea what they're talking about" so often.

[Rookie50]

It's amazing how pilots manage to make "you're absolutely correct" sound like "you're a complete idiot who has no idea what they're talking about" so often.

Why don't we all send you chocolates and a flowery card, along with an autographed copy of "The Aviator"

[photofly]

It wasn't me being quoted, but sure, if you like. Not liqueur chocolates though, I'm allergic to alcohol.

[Rookie50]

I like Turtles myself......

[Shiny Side Up]

This fatal spiral dive occurred in VMC, when the occupants of the 172 fell asleep.

http://www.tsb.gc.ca/eng/rapports-repor ... 8o0233.asp

The aeroplane struck the ground on an easterly heading, approximately 45º right bank, pitch angle 7º to 8º nose down, airspeed 140 knots.

What was most amazing about that one was that two of the three survived, the rear seat passenger with minimal injuries. A reminder that fatigue affects pilots of all sorts.

[pelmet]

"The Spiral Dive Conundrum

by Ron Rapp on June 8, 2015 in Instructing • 8 Comments


According to NTSB data, in the first decade of this century, over 40 percent of fixed-wing general aviation fatal accidents occurred because pilots lost control of their airplanes.

I find statistics like these mystifying. Why do people fumble the recovery from loss-of-control situations so often? After giving more than 2,500 hours of instruction, I haven’t noticed any particular trouble with it in training.

Perhaps that’s the problem: circumstances conspire to prevent accurate simulation of the scenario we’re trying to model. I’ve long harbored such suspicions, but could never really put my finger on exactly why. But in this month’s AOPA Pilot, Bruce Landsberg posits a theory that helps connect the dots:


Recovery from spiral dives, as presented in training, seems simple. Distract the trainee; place the aircraft into an incipient dive; have the trainee recover by immediately reducing power, rolling wings level, and returning to a level pitch attitude.

Unfortunately, this does not replicate reality. The trainee is primed to look for an unusual attitude, so distraction is absent. The recovery is started quickly for safety reasons, before the speed becomes a factor, so it’s a “baby” spiral — not its nasty big brother.

It’s difficult to induce vertigo under these circumstances, and true vertigo is a big distraction. In a well-developed spiral, the aircraft will quickly accelerate to well above trim airspeed.

…

As soon as the aircraft is rolled to level — assuming the pilot gets that far into recovery — it will seek trim speed. If cruise (and trim) airspeed is 150 knots and the pilot manages to get wings level at around 210 knots (and the maneuvering speed is 130 knots), there will be a pitch up as the aircraft seeks to regain trim speed. The pilot must push forward — firmly — to unload the aircraft structure.

Being so far above maneuvering speed, and possibly above redline, it’s not surprising that many inflight breakups follow a spiral. In some cases a trainee is told that since the aircraft is going down, they need to pull up. In a fully developed spiral, that’s exactly the wrong guidance! It’s going to be wings level and a push — if the aircraft continues to climb for a bit, getting away from Mother Earth is a great solution.

You’d think after 7,500 hours I’d have figured this out. But then, I’ve never been 50 or 60 knots beyond redline in a normal category airplane while trimmed for cruise.

As Bruce notes, the spiral dives we simulate in training are not like the ones encountered in real life. They can’t be, because we avoid exceeding Vne at all costs. It profits nobody if we cause an accident while providing the very training necessary to prevent one. Instructors are very much like doctors in that regard: “first, do no harm”.

So what’s the solution? Simulators? Unfortunately, even the most sophisticated Level D boxes — which cost millions (if not tens of millions) of dollars — can’t duplicate the loading a post-Vne spiral dive would generate. It seems to me that if the situation can’t be accurately modeled, we’re in no better of a position than with the baby-spirals we currently use in actual aircraft.

Regardless of where they’re held, these training events already lack the genuine surprise inherent in any real-world upset. When the instructor briefs a spiral dive recovery prior to the flight, you can bet your bottom dollar one might be in the offing.

Most pilots have never even been in a real spiral dive. These things are extremely dangerous. When I teach spins, I make a point of discussing and demonstrating the difference between spins and spiral dives. The primary difference, of course, is that spins are a stalled condition where excessive angle of attack and high drag keep airspeed low and stable. A spiral dive is an unstalled condition. Airflow over the wing is smooth, drag is low, and as a result the airspeed builds rapidly. Even to aerobatic airplanes, the spiral dive can be a fatally destructive event and I’ve seen people attempt a spin but not reach the critical AOA. The resulting flight path looks like a spin but is not.

Speaking of aerobatic airplanes, the latest trend is to utilize them for upset recovery training. This is a good thing, but spirals are a unique case. Aerobatic aircraft are better able to simulate the forces and sensations involved with a high-speed spiral dive, but the airframes aren’t like the ones we fly in instrument conditions. If you put 6g on an aerobatic aircraft, it won’t complain. The normal category ship, on the other hand, would be well beyond even the 50% safety factor of its 3.8g limit. The acro mount will also, among other things, feature larger and more responsive control surfaces, higher pitch and roll rates, lighter stick forces, and far less stability.

We can talk about a spiral dive all day long, but until you’ve seen one, I’m not sure the odds of recovery in a surprise situation are all that great, especially if vertigo is thrown into the mix.

If someone asked me how to best prepare for a scenario of that ilk, I’d recommend not even trying to simulate it with full fidelity. Instead, I’d suggest 10-12 hours of basic aerobatic training. Familiarity and experience in all-attitude flying goes a long way toward keeping panic at bay, bringing a sense of been-there-done-that to the unexpected. The load factors will be familiar. Higher than normal control pressures and/or deflections required for recovery will be as well. And most of all, staving off tunnel vision may allow the wayward pilot to recognize the increasing airspeed and figure out what’s happening before it’s too late.

It’s not a perfect answer, but at the moment it might be the best we’ve got."

http://www.rapp.org/archives/2015/06/spiral-dive/

[Rookie50]

Pelmet;

It's my view that some GA pilots have a challenge controlling their plane via handflying in IMC, especially perhaps slippery fast AC like the cirrus SR 22. Lose the AP and PFD, and some pilots would be in real trouble.

Even my 182; which is a pussycat handflying in IMC, I've noticed when practicing -- I use the AP for long trips -- its not hard to get tired and lose focus hand flying in turbulent (IMC), as it moves around a lot.

I'd imagine the SR 22, which I've briefly flown once, would be that much harder. Unlike my plane, you can't get slow in it, either, or it will bite. There are accidents where it appears the pilot just ran out of endurance trying to control the plane (on approach)

Think there is still value in partial panel practice, as well.

[pelmet]

Following up on my original post, I now have the similar Flying Magazine article....

"Revisiting the PC-12 Crash

Sometimes it's not the speed that breaks airplanes — it's the slowing down.

By Peter Garrison Posted August 24, 2015

In my March Aftermath column on the 2012 crash of a Pilatus PC-12 in Florida, I faulted the National Transportation Safety Board for mixing up indicated and true airspeeds. Actually, it was I who misread the report. I am indebted to reader Timothy Burtch, an accident investigator with the NTSB, for pointing out that the maximum speed of 338 knots that the airplane reached in a spiral dive before it broke apart was, in fact, an indicated airspeed, not a true one, and that the airplane did, therefore, exceed its maneuvering speed by 175 knots as the report stated.

The Pilatus, with a family of six aboard, was climbing through FL 250 in IMC. It was at 109 kias, in a 25-­degree right bank, deviating to avoid an area of rain, when the autopilot disengaged for unknown reasons. Presumably a chime sounded and a warning light illuminated, but the pilot seemingly did nothing to take control of the airplane. The baffling aspect of that accident was the pilot's apparent failure to act even when the airplane was vertically banked and plunging downward at a horrific rate.

Within 10 seconds of the autopilot disconnect, the angle of bank had increased to 50 degrees and the airplane had begun to descend. After 30 seconds, the bank angle was 100 degrees and the airplane had lost 2,600 feet. In the next 13 seconds, it lost another 5,900 feet while the positive load factor increased to 4.6 G. At 36 seconds the indicated airspeed, having peaked at 338 knots — 430 true — dropped to zero, suggesting that the breakup — the airplane lost its horizontal stabilizer and portions of both wings — had taken place somewhere around 15,000 feet.

Any airplane will enter a spiral dive, sooner or later, if no attempt is made to control it. Some may fly hands-off for many minutes in smooth air, but if they are disturbed by a gust, or if they are banked in the first place, they will inevitably bank more and more steeply, the nose will drop, they will pick up speed, and the turn will continue to tighten without limit.

Builders of free-flight models will counter that their airplanes can remain right side up indefinitely, but that is because they have much more dihedral effect than piloted airplanes do. If your airplane had the lateral stability of a free-flight model, it would be very reluctant to bank and very fatiguing to fly. For the sake of maneuverability, therefore, we accept the necessary evil called "spiral divergence."

In VFR conditions hardly anyone gets into a developed spiral dive unless he literally falls asleep at the wheel. It is the non-instrument-rated pilot who strays into a cloud who stands the greatest chance of experiencing a spiral dive. In this case, however, what made the sequence of events doubly puzzling was that the pilot was instrument rated and current.

The procedure for recovery from a spiral dive is simple: Power off, level the wings, slow down. But there are potential pitfalls, and the Pilatus may have encountered one of them.

The airplane was trimmed for 109 kias. The dynamic pressure of air at that speed is 39 pounds per square foot (psf). The wing loading of the airplane, which was probably close to its gross weight, was about 36 psf, and so the lift coefficient, which is the ratio between the lift force and the dynamic pressure, was about 0.9.

The dynamic pressure at 338 kias is 337 psf, and the lift coefficient in level flight, if level flight were permissible or even possible at that speed, would be about 0.1.

Normally we don't talk much about lift coefficients in the context of piloting, as opposed to designing, an airplane, but they are of interest in this case because G-loads can be thought of as the ratio of two lift coefficients. If, for example, the Pilatus is trimmed for 109 kias and a lift coefficient of 0.9, it would experience an acceleration of over 9 G's if its indicated airspeed were, by magic, instantly increased to 338. Another way to put it is that the acceleration is the square of the ratio of the high speed to the trimmed speed. Thus, if you were trimmed for 100 and you were suddenly indicating 300, the speed ratio would be 3 and the resulting G-load, as the airplane seeks to return to its trimmed speed, would be 9.

That is physically impossible, of course, because in the real world speed cannot instantly change from 100 to 300. But you don't need 9 G's to break an airplane, either. Six G's will do it, five in some cases, and that would have occurred at 270 kias or less. In other words, if you are trimmed for 109 kias when some sort of upset occurs, you pick up a lot of speed in a spiral dive, and then you level the wings and the airspeed indicator is still showing over 270 knots, you are entering the neighborhood of a normal-category airplane's ultimate load factor.

The ultimate load factor is not the one where you limp home with wrinkled wing skins. That's called the limit load factor. The ultimate load factor is the one where big pieces break off.

So a caution has to be appended to the recovery procedure for spiral dives. After you roll the wings level, the airplane is going to try to return to its trimmed speed. Normally, this will not be a problem. If you are cruising at 150 kias and you look up from a chart or your iPad to find yourself in a 60-degree bank and the airspeed indicator winding through 220, you are not going to take the wings off the airplane by just leveling out. You'll pull a couple of G's, perhaps, but nothing will break.

It's when you're flying at low speed, for example climbing, as the Pilatus was, and then get into a dive at very high speed, as it did, that the recovery may involve dangerously high load factors, especially if you try to help it along with a pitch-up command. What is actually called for is nose-down trim and a forward push on the yoke. Otherwise the airplane may overstress itself even with no help from you at all.

Inflight breakups are rare, but they occur regularly enough to be worth thinking about. Sometimes an airplane emerges from the bottom of the overcast already in pieces, but I suspect that some breakups occur only after the airplane emerges into the clear, when the pilot sees the ground in some highly unfamiliar position and instinctively overcontrols while trying to level out. This is a danger even when the overcast is quite high and there is ample room for a more gradual recovery.

A common piece of advice for VFR pilots who have blundered into IMC is to lower the landing gear. As a young pilot I used to wonder what in the world the landing gear could do to keep the airplane upright; the authors dispensing the advice evidently considered it so obvious that they did not bother to explain that the purpose was to increase drag and thus to limit the speed gain in the inevitable spiral dive.

In the extreme situation in which the Pilatus found itself, lowering the gear could be part of the spiral dive recovery: Power off, level the wings while trimming nose down, lower the gear — never mind the doors — and don't let the nose rise too rapidly. In a reciprocating-engine airplane, drag can be further increased by pushing the prop to high rpm.

The essential point, however, is to limit G while reducing speed. If the plane were going to come apart because of sheer speed, it would already have done so. It's the G-forces that pilots apply in their urgency to stop pointing downward that end up breaking wings."

[photofly]

It's when you're flying at low speed, for example climbing, as the Pilatus was, and then get into a dive at very high speed, as it did, that the recovery may involve dangerously high load factors, especially if you try to help it along with a pitch-up command. What is actually called for is nose-down trim and a forward push on the yoke. Otherwise the airplane may overstress itself even with no help from you at all.

You heard it here, first, folks:
http://www.avcanada.ca/forums2/viewtopi ... 38#p931710

[PilotDAR]

... and then get into a dive at very high speed, as it did, that the recovery may involve dangerously high load factors, especially if you try to help it along with a pitch-up command. What is actually called for is nose-down trim and a forward push on the yoke. Otherwise the airplane may overstress itself even with no help from you at all.

I find this worrisome. Who is calling for a forward push on the controls during a spiral dive recovery?

There are certain things in piloting an aircraft, which if left unattended for too long, will be fatal. The pilot must do some things at some times to prevent disaster - there's just no safe alternative. Among these things which demand that the pilot actually fly the plane, is preventing uncontrolled acceleration during high speed dives.

The drag increase resulting from pulling G is a small part of preventing an even faster speed build up, but you're in a very dangerous place if you're pulling significant G above Va - particularly without a G meter! - Just don't go there!

It is not an entitlement of piloting that you will be kept out of this dangerous realm, and certainly an autopilot is not assurance either. Somethings as the pilot you are responsible for, and averting dangerous dives is one - no excuses.....

[photofly]

... and then get into a dive at very high speed, as it did, that the recovery may involve dangerously high load factors, especially if you try to help it along with a pitch-up command. What is actually called for is nose-down trim and a forward push on the yoke. Otherwise the airplane may overstress itself even with no help from you at all.

I find this worrisome. Who is calling for a forward push on the controls during a spiral dive recovery?

Everyone who who has the slightest understanding of the flight characteristics of an airplane flying at three times its trimmed speed. That includes you, I very much hope.

Let's talk of elevator trim (again) and why he speed builds up - hands off -in a spiral dive.

Here's a thought experiment: trim your 150 for 40 kts (or as slow as it will trim) then accelerate to 120kts (in a dive if you can't get to 120 in level flight.) Which way are you pushing or pulling on the yoke? What will happen if you let go? What will happen if you pull back?

Now the hard part, for the good students: why does an aircraft fly hands off faster than its trimmed airspeed while in a 75 degree bank? Exactly how much faster? At what precise point in the recovery from a spiral dive does this particular effect cease whereupon we find ourselves in the same sort of situation as the pilot in the previous paragraph? What should we (just like the pilot in the previous paragraph) do with the yoke? What will happen if we don't?

[cgzro]

Seems the proper and easy to agree to method is to level the wings and ensure the pitch up stays below your ultimate G. Hard to argue with that right?

Now that could be with trim alone, trim plus pull up, or trim and push. It all depends where the the trim is and your speed. In other words the trim alone at some speeds is capable of generating your max +G or more (hence a push may be required).

One of the answers given was to go nose down trim and then push. It would seem to me that if you have removed the trim then pushing is no longer necessary and you need to be pulling once that trim is off. If however the trim takes a while to come off (which it always does) then of course you are pushing for a while as it comes off to keep the G where you want it and then you need to start pulling once the trim is more neutral.

In other words without a G meter you need your butt to tell you where 4-5G is and you need to keep that throughout the recovery. Without experience in what 4-5G feels like I'm not sure how you can execute this optimally and given the choice of over / under speed or over / under G you are probably better of over speed.

Yet another good reason for unusual attitudes. To know what 5G feels like.