Is it practical to intentionally stall an aircraft to execute a fast, controlled descent?

Question

An MD-11 had an onboard fire, but had to request more flying distance at least partly to execute a safe descent (and to dump fuel). It ended in tragedy.

In another accident, an A330 descended 30,000 feet in three minutes, in control (or at least in a recoverable state) during a stall.

Right now, airplanes in general, and particularly large jets, seem to be limited to 8000 fpm. So, I am wondering, can an intentional stall really accelerate this descent in a safe manner? The very high drag would keep the airplane from overspeeding, and there's no need to certify things like thrust reversers for deployment during flight. Heck - it could even be used to speed up the approach phase of a landing, saving time and airspace.

Answer 1

It isn't practical for a number of reasons:

1. Intentional stalls are inherently dangerous. Stall-spin accidents are a major cause of accidents, stall recognition and recovery are taught specifically to avoid stalls. Some airplanes have docile stall characteristics, but even those can still bite you. A Cessna 172 will drop a wing if mishandled, especially when heavy. Stall behavior at high altitude can be very severe
2. Some airplanes with a t-tail can enter what is known as a 'deep-stall', where turbulent airflow masks the tail and makes the elevator ineffective and the airplane unrecoverable
3. Situational awareness: dropping like a stone with the nose up does not give pilots a view of the outside, or see what mountain they may be heading towards
4. It takes time to do a controlled stall. You have to pull throttle and maintain altitude, degrading airspeed until you enter a fully developed stall. This isn't instant, in a light airplane it will take 30 seconds or so, I'm not sure how long it would be for an airliner at cruising altitude, but I imagine it may take a minute for a fully developed stall to develop and to get that high rate of descent, whereas you can be in a dive with a steep rate of descent in a much shorter time

Consider an emergency descent from FL400 (40 thousand feet) to FL100 (10 thousand feet) using the 2 methods: A dive at 8000fpm and a 'controlled' stall at 10000fpm, where it takes 30 seconds to get into a developed stall and high rate of descent:

• 60 seconds: the diving airplane is at FL320, the stalling airplane at FL350
• 2 minutes: the diving airplane is at FL240, the stalling airplane at FL250
• 3 minutes: the diving airplane is at FL160, the stalling airplane at FL150
• 3 minutes 30 seconds: the stalling airplane is at FL100
• 3 minutes 45 seconds: the diving airplane is at FL100

So, by these calculations the stall option saves 15 seconds to lose 30 thousand feet because of the time penalty to get into a stall in the first place. That's really not much, and considering the risks involved from the potential loss of control and spacial awareness it's simply not worth it.

Not only is it not practical, it's not necessary to descend faster. A high speed descent is required for a number of emergencies, a cabin depressurization being the most prominent one, but there you have emergency oxygen supplies which will last much longer than required, so a few seconds will make little difference to survival. With an in-flight fire or other emergency where landing quickly is imperative the limitation is not descent speed, but proximity to landing fields which determines survival.

Answer 2

You don’t need a new technique

You don’t normally stall jetliners. And in a crisis the last thing you want to do is learn a new technique.

Besides, they already have a trained practice for descending jetliners very quickly. It’s used for loss of cabin pressure. While the procedure normally levels at 10,000’/3000m, it could certainly be extended.

Anyway, descent time has not been the failure point

When you look at fire-on-airplane tragedies, the pinch point has not been inability to make bare-minimum time to landing/evac. The problem has been pilots overestimating the time they have available.

Look at the UPS Air tragedy and go figure where they were in the airspace at the first sign of fire. The closest large airport with facilities was an Iranian military base on a nearby island. The second closest large field was Doha, Qatar. They decided to return to Dubai.

Now, think about that decision-making. Iranian military base with a US-flagged airliner technically in the US military reserve? Yeah, not an ideal situation. Doha, this was obviously a maintenance problem and they didn’t have a maintenance base there. Dubai was only, what, 10 minutes further.

And of course, as is very typical in these flights, they continued moving away from Dubai after the first sign of trouble, until they chased the problem a bit and concluded yeah, they really did need to return.

So we have a number of potential saves on the table, but they did a careful analysis and made a more preferential choice given a number of factors. The time spent making those choices sealed their fate.

Then we look at the flight that went down off Halifax. Again, the first sign of trouble, and indeed the second sign of definite fire, happened fairly early - and they still had a variety of divert airports (some not very good choices for a fuel-heavy jetliner). They chose Halifax, which could’ve worked if they had realized they were on the clock, but then they piddle-diddled in the air with a great deal of time-wasting communications and procedural stuff, until they were overcome.

Why not just beeline for the nearest airport?

The problem is that, early on, the signs are ambiguous. Those early signs are often seen without an ensuing fire. So the pilots are concerned with being perceived as alarmist or “making the problem up” if it turns out to be nothing special.

And a divert complicates things. The descent, landing, takeoff, climb and resumption of flight takes fuel that wasn’t allocated, which means the turns-out-to-be-nothing aircraft must buy fuel at the divert airport - where they may not have a supply contract.

Either company or a perhaps non-contract FBO (independent mechanic) qualified on that type must be found and/or flown into that airport, to clear the plane for continued flight. (If it’s nothing). Meanwhile the added time will put this aircrew “over time” so they will not be able to finish the flight. New aircrew must come in also.

Likely the most expedient way to get people on their way is for the airline to bring in another heavy, and pay more landing fees at an airport where they do not have a contract, other aircrew called in, etc. This may take enough time that the airline has to book every hotel in the 3 nearest towns, and find transport for them all, which basically involves getting someone from the local middle school on the phone on a Sunday.

Like the Halifax case, it may involve hundreds of people going through immigration in a country they might not have a visa - heck, it creates another crisis if they land a 300-pax jet at an airport with no Immigration facilities (or a small one sized for 12-passenger rubberband planes). Remember they “skidded to a stop” on the runway, deployed slides, and pax fled the aircraft, and are all over the airport, hopping the fence to seek help at neighbor’s homes, even venturing into town. Hey, that family that US CBP was deporting, escorted onto the plane and stayed until we pushed back - has anyone seen them?

So diversion creates a Big Mess, and needless to say, the pilots do put some energy into avoiding it. And that means time... tick tock, says the fire.

Answer 3

Normally, a stall and controlled flight are mutually exclusive.

That AF447 would descend as it did has to do with the relaxed static stability of the A330 and its rear cg location as well as the docile behavior of its airfoils with large separation on the upper side. In short: With some aircraft this is indeed possible and practical but with others it is highly dangerous.

This technique is even promoted by W. Langewiesche and called "mushing glide". To quote from my answer to that question: "I would only fly this after extensive stall testing at altitude, calm winds and when I know exactly how the airplane reacts to control inputs.".

Answer 4

It is smarter to roll onto your side and maintain unstalled flight, executing a emergency spiral descent. G loads on the wings are much lower as there is no need to maintain altitude, only to control airspeed.

With all due respect to our beloved Langewiesche, "mushing glide" technique is for much lower wing loaded gliders that are easily unstalled with a forward push on the stick. The pilot is greatly aided by a stall warning buffet and is able to react in time.

To truly understand stalling a much higher wing loaded airliner, make a model out of lead. Yes, it will fly at a high enough airspeed, but full stall recovery will take thousands of feet, and is there for an impractical technique. Jet airliners need to keep moving.

would high drag keep the plane from overspeeding?

Leaf - yes
Langwiesche's personal glider - yes
Lead model glider - no
400 ton airliner - NO

Finally, one needs to consider a rate of descent of 10,000 feet per minute is over 100 mph straight towards the ground. The physics of bringing 400 tons to a halt (zero rate of descent) leads to the conclusion that 8000 feet per minute is adequate.

Answer 5

Yes - and practical enough it could save your life.

Spinning - which is a certain type of stall - was invariably fatal in the early days of flying, with 90% of fatal accidents attributed to this cause.

One of the first recoveries from a fully-developed spin was Lt Wilfred Parke RN in 1912 in an Avro G. It was written up for Flight magazine, and the manoeuvre became known as "Parke's Dive", a magazine article that became well remembered, as attested by J A Chamier during WW1 -

Maj J A Chamier who, while in France, found himself spinning as he came from a cloud. While he was falling, he recalled an incident on Salisbury Plain before the war when Lt W Parke RN had recovered from a spin near the ground. When people had crowded round to congratulate Lt Parke on his luck, he had explained that he had stopped spinning by doing ‘everything wrong’. Maj Chamier likewise did the opposite of what his experience as a pilot suggested and he also recovered. He subsequently related his adventure to Royal Flying Corps head-quarters. [Jones, H A The War in the Air, vol 7 (Ch VIII, 430/431)]

As R.F.C. pilots began returning from the Western Front to become flight instructors they brought with them experience in spin recovery. In the turmoil of air warfare, recovery procedures had been found by trial and error as a necessity for survival. Spinning commonly followed the disorientation that often occurred when pilots (without guidance from instruments) had to climb through cloud or enter cloud to escape interception, as in the experience of Maj Chamier above.

This experience was highly pragmatic: its benefits were both as a manoeuvre itself, and as a ruse to fool the attacker into believing their opponent had lost control. And so, according to multiple accounts, intentional spinning and its accompanying rapid descent became a deliberate, practical manoeuvre with uses in air combat.

(Royal Flying Corps officer running a class on attacking manoeuvres aircraft can perform. source)

(Principal Source - B J Brinkworth On the Early History of Spinning and Spin Research in the UK Part 1: The period 1909 - 1929 Journal of aeronautical HIstory Paper No. 2014/03)

Answer 6

The other answers all seem good, but I think are not complete. And Isay that with the utmost respect for the others who replied, most of whom are without doubt much more experienced pilots than me.

First point, and perhaps was mentioned, but when you are touching down, you do in fact intentionally stall the plane, or at least in small prop planes. I only have a license for them, so am not sure if it's the same for jets.

But in effect, there are two extremes of fixed wing plane. The first type has a high glide ratio: if you lose thrust, your lift is still quite close to weight. That's most prop planes. In fact, the upper limit of this is powered gliders, that don't actualluy need the thrust after take-off. The second has a very poor glide ratio, and as soon as you reduce thrust, you might not stall, but the lift is significantly reduced, and your vertical velocity becomes quite large. This is true for all jets: commercial and military.

In terms of the practicality of an intentional stall, if you are not intending to get lift anymore after it, or even turn, then there's not that much downside to it. How can there be if it's the standard procedure for touchdown in all fixed wing prop planes?

And in terms of killing your lift, without increasing horizontal or vertical velocity components, in a fixed wing aircraft, I don't see anything you could do that would be BETTER than intentionally stalling. Except with a special plane like a crop duster, or STOL (Beaver etc), with huge flaps, that's not the case. Or with a plane where you can adjust your thrust, like with a Harrier, or Osprey.

In any case, the question is only about a LANDING (as in it doesn't specify the plane must take off again). I'm not sure what you were told, but my instructor told me any landing I could walk away from was not a failure, per se. So in some special case, maybe it's the best thing to do, to allow a walk-away?

And it doesn't say it needs to be with a commercial passenger jet either. I've not read all the replies, but I think they all assume too much, and don't think broadly enough. It is a thinking-out-of-the-box type question. Perhaps there could be specific circumstances where it makes sense.

As you all know, a fixed wing can stall at any speed, and any attitude, if you cause the airflow over enough of the lifting surfaces to stop creating lift. That's the definition of a stall, in any case. And a stall as it starts, could well be the best/fastest way to change your flight vector, if you are constrained in terms of time, engine thrust, or distance (horizontal and vertical).

If one assumes the plane was flying straight and level, not climbing, and at some height where it couldn't immediately touch down, then you'd need to specify or know the exact parameters about its flight condition to be able to answer this properly.

Obvously it's not the ideal procedure, because it usually leads to a loss of control with most conventional planes, but if one were to add more flaps on a current design, maybe it could indeed be of use.