Superstall recovery

[merchants]

I’ve been reading a book on the old HS Trident, and it says that a prototype was lost during testing due to a ‘superstall’, which the aircraft entered inadvertently. Sadly, all on board were killed.

I understand that such a stall is unrecoverable because the disturbed airflow from the stalled wings flows over the elevator in a T-Tail aircraft. The Elevators thus cannot be used to recover the aircraft.

But would it be possible to use the ailerons to recover? I read somewhere else that the secondary effect of ‘roll’ is ‘yaw’. So if you rolled a ‘superstalled’ Trident, would the nose drop due to yaw, as the aircraft rolled, thus enabling recovery?

Or would the disturbed airflow over the wings prevent such a manoeuvre?

As a layman, I’m interested in this. After all, the worst aircrash on UK soil was a ‘superstalled’ Trident that crashed at Staines in 1972, killing all 112 on board. There are still T-Tail designs flying (e.g. Gulfstream, Boeing 717), and I wondered if this had ever been considered?

[BlueEagle]

Any BAC people out there? After a BAC1-11 was lost on Salisbury Plain, due to a super-stall, it was my understanding that a parachute was fitted in the tail cone as a last resort, should it happen again.

Again, my memory is not up to much but, I believe in the case of the BAC1-11 the test pilots tried everything they could think of on the way down and transmitted everything they were doing?
(This would have included rudder and aileron inputs, I am sure).

[Mad (Flt) Scientist]

merchants:
But would it be possible to use the ailerons to recover? I read somewhere else that the secondary effect of ‘roll’ is ‘yaw’. So if you rolled a ‘superstalled’ Trident, would the nose drop due to yaw, as the aircraft rolled, thus enabling recovery?

As you noted, the airflow on the wings may prevent the ailerons being effective. If the aircraft has deep stalled (or "superstalled") chances are it is a tip stall, which would cause a pitch-up for a swept wing plane. So most likely the outboard wing is pretty much separated. In those circumstances the aileron power will be minimal. Any over-wing surfaces (spoilerons) are also likely to be ineffective in generating roll.

More to the point, attempting to recover from a stall by generating yaw/sideslip is asking for spin entry; which isn't much more survivable in a T-tail transport than the stall was.

What you need is a means to generated a nose-down pitching moment - landing gear extension is one means, "anti-spin" or "recovery" parachutes are another. While it is certainly worth trying almost anything in that situation, I doubt that inducing yaw is the best option.

[knobbygb]

There do seem to be a lot of modern passenger carrying designs about at the moment with T-tails (I'm thinking specificaly of the Bombardier and Embraer RJ's) - I assume 'deep stall' must have been investigated for these a/c (in a wind tunnel, I hope:eek: ).

Just what are the chances of one of these a/c being lost due to this? Is it a situation pilots have to be particularly aware of or has it been 'designed out' in some way? Is it more or less likely to be a problem on a smaller aircraft such as this than, say, on somthing bigger like an MD80.

[Mad (Flt) Scientist]

Mike Jenvey
surely it is a certification requirement these days to have a "stick pusher." In other words, after the stick shaker warning approaching a stall, the pusher operates before the stall, hence stopping the aircraft getting into a super-stall condition

There is no certification requirement for a "stick pusher". FAR 25.203 merely identifies "A nose-down pitch that cannot be easily arrested" as one of the possible stall indications.

Not all T-tail aircraft by any means have pre-stall pushers (in the sense of being pre-aerodynamic max lift); in several cases the pusher is set to fire shortly after the aerodynamic stall.

And there is nothing to stop you having a T-tail with purely aerodynamic stall warning. It is often simpler to define the stall via a pusher than through aerodynamic devices (stall strips etc) but that is a technical decision left to the OEM.

There is also no certification requirement which specifically addresses "superstall" or deep stalls. It is only implicit in the requirements.

[exeng]

<<Don't know about the Trident, but surely it is a certification requirement these days to have a "stick pusher.">>

The Trident did have a stick pusher system . In the 'Staines' accident the crew elected to overide it. There had been a lot of 'history' on the fleet at that time of 'false' stall warnings. It was presumed that the crew in this instance believed that this was yet another false warning and overode it to prevent operation of the stick push system.


Regards
Exeng

['%MAC']

Deep stall, or super stall is not confined to T-tail aircraft. The F-16 will deep stall at alpha = 50. That aircraft uses both an alpha and roll limiter to prevent the aircraft getting into deep stall.

The F-16, as many aircraft, exhibits a non-linear variation of pitch moment with alpha. This provides two stable trim points, the conventional trim point at low AOA below the stall, and a poststall trim point, which is the deep-stall. Due to stability problems, aircraft without computer aided flight controls have a problem getting AOA high enough to get into deep stall – a good thing.

[merchants]

Hmmmm........Didn't consider that you may induce a spin entry by 'rolling to yaw', but now you come to mention it, that does sound plausible.

But how would 'lowering the gear' help. Do you have a 'nose heavy' condition when this happens?

I suppose in that situation, you'd be doing ANYTHING to recover!

Didn't know also that we lost a 1-11 due to this. Maybe that's why our CAA insists on 'stick pushers' for these types of aircraft?

[PPRuNe Pop]

The first example of a deep stall was, as Blue Eagle says, over Salisbury Plain in a BAC1-11. It was flown by Mike Lithgow, a brilliant test pilot. He 'talked' on the RT all the way down, as they tried everything to correct it, passing on every piece of information they had. I heard the whole conversation at a GAS meeting some few moths after. Quite remarkable.

Stick pushers and shakers were introduced later, but not before experiments, mostly successful, were made with tail parachutes.

[Mad (Flt) Scientist]

But how would 'lowering the gear' help. Do you have a 'nose heavy' condition when this happens?

It might help (and only 'might') if the drag of the gear below the c.g. were enough to nudge the aircraft a little nose down AND you only needed that little nudge to get some tail power back.

[ronnie123]

Some help here pls...I may sound like a flamed out engine any way here goes...

What does the superstall look like ? What are the most imp features in such a stall.

thks:confused: :confused: :confused: :(

[john_tullamarine]

Ronnie 123,

Several things to be aware of

(a) "normal" 1g certification stall .. slow approach for certification numbers and not of much real relevance to the day to day world

(b) "normal" pilot type stall which tries to a greater or lesser degree to emulate (a)

(c) stall under acceleration loading ... often called a g-stall. If the rate of approach to the stall is moderately high then the increased load factor will result in a stall at a higher airspeed than for (a) or (b). Unsteady flow characteristics, especially with flap out, can make for some exciting departures during the stall and recovery.

(d) "deep" (what this thread is calling "super") stall. (With a T-tail configuration particularly) some aircraft can find themselves in a situation where the wake from the wing and hull "blankets" the tail. If this also results in a significant loss of nose down pitch capability/authority, then the pilot may be in a position where the aircraft is not able to be unstalled unless he/she is able to find a combination/sequence of control and thrust inputs which can get the aircraft doing something more useful than just going down .... and this could take some considerable time to achieve. Alternatively, in the test environment, a conventional anti spin chute might be used to provide a nose down pitching moment to help things along with the recovery.

There have been a few interesting reports in the literature regarding some of the antics which the FJ community need to be able to do to achieve recovery and these reports might be of interest to you.

(e) accelerated stall. If the pitch rate is very high, there is a phenomenon where the flow can reattach under the influence of a spanwise vortex and the wing keeps working at alpha much higher than normally experienced. Not of much interest to the routine FW community but can be important in the RW world.

[GlueBall]

You could try using asymetrical reverse thrust to raise a wing which would help to get the nose down. In airplanes where reverse thrust is locked out in flight, the gear could be dropped and groundshift circuit breakers could be pulled to effect ground logic, then a reverser could be deployed up to max reverse thrust. I recall that the DC8 had normal inflight reversing capability on the inboards.
It would be, as was said, a last ditch life and death effort for survival.

[spekesoftly]

I believe the Trident could quite happily use reverse thrust in flight, and achieve some very high rates of descent, in so doing. However, with its tail mounted engines, I expect reverse thrust would probably have exacerbated a deep-stall condition.

The incident referred to at the beginning of this thread happened in the Summer of 1966, and the Test Pilot was Peter Barlow.

The BAC 1-11/ Mike Lithgow incident, although earlier (1963), was most definitely not "the first example of a deep-stall". The loss of a Gloster Javelin, in June 1953, was attributed to 'super-stalling', and doubtless there were other earlier examples.

[747FOCAL]

I am not a pilot, but I have been in over 600 heavy aft cg stalls. The majority in a 727 certifying hushkits. What I was told is that once you "blank" the tail on a T tail aircraft you are usually dead. The aircraft will flip. If the pilots are still alive and capable recovery is still possible unless the aircraft has sustained unrecoverable damage.

Alaska 261 lost the stab when the pilots activated the auto pilot which released the brake. The aircraft(md80) flipped end over end until it hit the water at which point it was backwards.

If you look at Md80 and 727 FedEx hushkit planes you will see the "fins" on the engines that are there to energize the aft turbulent air and hopefully keep the tail from "blanking: and allow the pilot to recover.

Boeing calls them "Chines" If you imagine the trajectory of a plane in stall you can see how these might help.

[chiglet]

In the "Javelin", [a 1960s-'70s RAF Fighter]
IF said a/c entered either
a, an incipient spin, or
b, a "stable stall".
The only way to recover was for the Nav to "Bang Out":eek:
we aim to please, it keeps the cleaners happy

[BlueEagle]

The Gloster Javelin was a bit earlier than 1960/1970, I made the KeilKraft model of it around 1955!

I think it is correct to say that the BAC1-11 was the first CIVIL aircraft to be recorded as having crashed through a deep stall.

[chiglet]

Soreeeee!!:confused: :o :o :o
I "meant" to say 1950-60s.
"Wnen I wuz in Borneo" etc, etc.
The [ex] Jav Navs were "waxing poetic" about the "Navs [lack of] canopy giving enough drag to dig them out of the S/mire
we aim to please, it keeps the cleaners happy

[henry crun]

chiglet, The Javelin spin characteristics were different to those you describe.
There was no incipient spin or stable stall with that aircraft, if stalled inadvertently, and only a fool would do it deliberately, the aircraft would spin.

The spin was unstable, in fact even the direction was unpredictable even from turn to turn, and the nose would pitch up and down through 70 degrees.

Recovery was also unpredictable, taking one of two forms, and involved a large loss of height, but was not impossible.

I heard of no cases of the nav ejecting affecting recovery, if this did happen it was more likely as a result of the pilot persisting with the recovery action, which could take some time, after the nav had left.

[steamchicken]

There was a detailed Tech Log thread on deep stalls, and here is the link (http://www.pprune.org/forums/showthread.php?s=&threadid=9302&highlight=Deep+Stall). Interesting that apparently the VC10, far from pitching up into the stall, pitches down despite being a T tail. Seems to have the full monty on the issue

[avialuver33]

In the book "Air Disaster, Volume 1", Macarthur Job provides a fairly detailed account of the Staines accident. As stated in an earlier thread, the crew did, indeed, disengage the stick pusher. The article goes on to state that, via FDR data, the cause was traced to retracting the leading edge droops at too low an airspeed. Figures in the article are listed as 162 knots @ 1770 feet when droop retract occured.

What I find curious about this accident, is the number of qualified personnel on the flightdeck at the time. The text lists captain, first officer, a second officer in the monitoring pilot's position, and a BEA Vanguard captian, who was qualified on the Trident, in the jumpseat behind the pilot. I can't help but wonder why no one saw the premature droop retraction.

Further along in the same book, another deep stall, this one in a Boeing 727-200, is recounted. This accident was apparently caused by an iced up pitot head. CVR transcripts point to a failure to activate pitot heat during the pre-flight check, which lead to erroneous rate of climb and airspeed indications. At one point, readings indicated a 5000 fpm climb at 340 knots. Thinking that the spirited performance was due to a light aircraft (ferry flight), the crew pulled back on the control column to prevent excessive airspeed. At around 24,800 ft, 30 deg nose up attitude, and 165 knots, the stick shaker activated. Believing that the airspeed readings were correct, the crew mistook the stall warnings for Mach buffet. The text goes on to state that the aircraft stalled, and the disturbed airflow over the wing blanketed the elevators, rendering them useless. Apparently, this phenomena is peculiar to the configuration of T-tail aircraft.

Both accounts made for some very chilling reading.