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bobrun
27th Nov 2004, 22:50
Does anyone know why the recovery technique of a stalled horizontal stab (icing conditions) calls for pulling on the yoke?

It would seem that pulling on the yoke would only increase the AoA of the stalled horiz. stab...:confused:

Looked everywhere and I can't find a detailed answer.

PickyPerkins
27th Nov 2004, 23:18
I have never flown with a stalled horizontal stabilizer, and it sounds to me like a maybe non-recoverable situation with uncontrolable increasing pitch down. But here is my guess.

For a plane flying straight and level the lift on the wings is up while the lift on the horizontal stabilizer is down. This is required for pitch stability, and is a consequence of the center of lift on the wings moving forward as the angle of attack increases. Without the downward force on the tail the aircraft would be unstable in pitch (unless it had a canard).

So if the tail stalls due to icing the downward "lift" needs to be increased somehow, and I guess up elevator might be one way of doing this. Pulling on yoke should decease the AoA on the horizontal stabilizer

Maybe someone who really knows the suject can expand on this.

Cheers,

http://home.infionline.net/~pickyperkins/pi.gif

batty
27th Nov 2004, 23:33
In straight and level flight the lift on the horizontal stabilizer isnt always down , it depends on where the CofG is. In order to increase range an aft CofG is preferable in the cruise so that there is lift on both the wing and the stabilizer. If the stabilizer is providing a downward force it in effect acts as an inrease in aircraft weight and thus a decreased range. An aft CofG does decrease the longitudinal stability though.

27th Nov 2004, 23:36
You need to somehow generate the required amount of lift on the tail surface to trim out the tail-off pitching moment. If the ice on the tail leading edge is causing the tail to stall, you might - and only might - be able to increase the elevator angle (by pulling back on the stick/yoke) and also decreasing the actual stab angle - that way you might manage to give a more aft-loaded tailplane lift distribution and so unstall the tail.

The problem I can see is that the process of unstalling a surface is not the reverse of stalling it - usually there's some hysteresis - so you'll likely have to reduce the AoA at the tail to somewhat lower to unstall the tail than the angle it stalled at.

That would seem also to be quite a slow process - trying to find a new STAB/elevator combination isn't immediate, and if the tail is meanwhile stalled other, bad, stuff will be happening.

If pulling back on the yoke does something - and I dont see how it could be guaranteed - you might be able to start a niose-up pitching motion. The dynamics of the plane's rotation would tend to reduce the angle-of-attack on the tail, which might help unstall the tail, giving long enough to input a nose-down stab input.

It all sounds very very dicey though.

oldebloke
28th Nov 2004, 02:50
Interesting concept....The first thing to call/select ,in the event of tailplane stalling,is to reduce flap setting(back to where they were)as the selection of the flaps rediected the airflow into the stall angle...Viscount/DC4 cases...
As has been said before the increase in backstick can only increase the curvature of the down lift stabalizer..:ok:

Milt
28th Nov 2004, 03:37
Horizontal tail stalling is a very rare event. Perhaps we don't hear about it because it is most likely to have terminating effects.

Ice accretion on the wings will most often occur and be a problem before enough builds on the tail. When it does build up on the tail the ice will be depleting the down lifting characteristics of the tail surfaces rather than causing them to stall in the usual sense. So with ice build up on the leading edges you should still have some control with the trailing elevator/s and also even if it is a full slab tail.

Incidently FBW and artificial stability will be a way we can design for a zero tail load or even an up load on the tail for cruising flight..It's happening with military aircraft and close for non-military using artificial stability. But the elevators will then have a very busy time preventing runaway in pitch.

Can anyone describe an actual case of horizontal tail stall or major loss of tail downforce resulting from ice build up before it becomes a major problem with the wings.?

Maybe on an aircraft only fitted with wing de-icers.

alf5071h
28th Nov 2004, 10:48
bobrun – ice in the Cayman Islands? But seriously we must not forget ice at higher altitudes and in all parts of the world.
The main difference in recovering from a tailplane stall in icing conditions is that generally these stalls are ‘negative’ stalls, thus the control inputs are reversed.
My reference for the following is “Think Ice 2000” from BAE SYSTEMS.

A negative tailplane stall is more associated on aircraft with mechanical elevator controls.
Normally, the tailplane creates a force (lift) in the downward direction to balance wing and fuselage pitching moments. Under normal conditions, without ice accreted, aerodynamic pressures above and below the elevator are roughly equal and thus create no significant control surface hinge moment.
Extending the flaps increases the airflow downwash angle from the wing and increases the tailplane AOA more negative. For a given flap setting, the AOA on the tailplane becomes more negative with increasing speed because of the reduced AOA of the wing (more nose down, more tail up).
Therefore, at higher flap angles and high airspeeds the wing stall margin is increased, but the tailplane stall margin is reduced.
The occurrence of stall on any aerofoil contaminated by ice almost always occurs at a lower angle of attack than a clean aerofoil; hence any ice accretion on the tailplane reduces the tailplane stall margin further. It is worth emphasizing that ice can form on the tailplane at a greater rate than the wing, primarily due to its relative small size and smaller leading edge radius. This can lead to a significant build up of ice which is not evident from observation of ice accretion on other areas of the airframe.

From the pilot’s point of view, the most important characteristic of a tailplane stall is usually the suddenness and magnitude of the nose down pitch change most often accompanied by unusual stick forces, with the control column moving towards the forward limit.

Link to a larger extract and diagram from “Think Ice 2000” (http://uk.geocities.com/[email protected]/alf5071h.htm) : see Tail Ice / Tailplane Stall.

The FAA has a good training aid / video; I don’t have a link, but it was distributed on a DVD.
Edit: Delete reference to FAA DVD; substitute NASA Multimedia CD-ROM "A Pilot's Guide to In-Flight Icing" NASA Glenn Research Center, Icing Branch, 21000 Brookpark Rd. MS 11-2, Cleveland, OH 44135, http://icebox.grc.nasa.gov

411A
28th Nov 2004, 17:44
Yes, Milt, I believe I do recall an incident of tailplane icing, which resulted in the loss of the aeroplane.

IIRC, it was a Swedish(?) registered Vickers Viscount, on approach...some time ago.
Apparently the Viscount suffered from this problem under certain conditions, and the new operator was unaware of the previous difficulties.

I do personally know that the DC-4 had similar difficulties, altho to my knowledge, no aircraft was lost due to this.
When the DC-6 was designed, the same wing (with very minor increased washout toward the tip) and horizontal tail surfaces were used, but the DC-6 had combustion heaters for anti-icing, which were very effective.

John Farley
28th Nov 2004, 19:20
In the early days the Hawk could suffer from tailplane stalling gear and flap down given a tad of ice (found in Finland) as has been said pulling up the flap is favourite (it certainly can't do any harm) The prob was fixed on the RAF standard of jet by removing an inboard section of the flap vane (a type of slotted flap was used) This put up the approach config stalling speed a tad but there was a lot in hand on the RAF spec so no sweat.

Later when more lift was needed on heavy external stores overseas versions and especially the USN T-45 some strakes fixed to the fuse just below and near the LE of the tailplane (think of them as huge VGs if you will) reduced the local AoA of the tailplane and fixed the problem allowing the restoration of a full span flap vane.

blackmail
29th Nov 2004, 00:32
hello every one

the horizontal stabilizer is basically a wing flying upside down with the lift vector pointing downwards, assuming the cg is forward of the center of pressure. so in case of tailplane stall you have to use opposite techniques to recover: gently pull on the yoke & slowly add thrust. some accidents happened when selecting full flaps with a violent & uncontrollable pitch down due to tailplane icing. viscounts had this problem as stated in another post. i guess that if you change configuration & the aeroplane has an unexpected reaction, immediately go back to the previous flap setting & use above mention recovery techniques if tailicing is suspected. read an article about this long ago, where this was flighttested with a twinotter.(scary!). pitchanomalies & unusual vibrations in icing conditions all can point to tailicing.
always wondered why b737 has no tail deice/antiice protection devices. boeing says it is not needed, well?

oldebloke
29th Nov 2004, 01:21
Cases in point: DC4 Slick Airways,10mar64.Boston-"Loss of balancing forces on stabalizer, due ice accretion,causing aircraft to pitch nose down too low to effect recovery..
Viscount,Skyline sweden,15jan'77-"ice on leading edge of Stabalizer causing flow seperation and Stabalizer stall..
Viscount,Capital airlines06Apr58,Michigan,Undetected ice accretion on stabalizer,in conjunction with specific speed and configuration,caused loss of pitch control... :hmm:

Astra driver
29th Nov 2004, 01:25
We are usually taught tail stall recovery at every other recurrent session while attending Flight Safety at ILG. (Wilmington, DE USA)

We are told that a tail (Icing) stall will usually occur at higher airspeeds while deploying flaps as opposed to the wing stall, which would, of course be at lower speeds and in any configuration.

The stall, when it occurs is identical to wing stall, the only clue to the diffference being the circumstances under which they occur.

The recovery procedure is to apply max power and pull back on the yoke while returning the flaps to their previous setting.

I think that if you are in potential icing conditions and prime yourself for the possibility of a tail stall you should be able to recognise the onset before it becomes a problem.

411A
29th Nov 2004, 01:36
Oldebloke was kind enough to provide the references needed, so it sure does seem to be a problem on some particular types.

Similiarly, as blackmail mentioned with the B737, the Lockheed TriStar does not have tail anti-icing either.

Altho the TriStar is a unique design amongst civil jet transports with its 'all flying' stabilizer, I would suspect many jets do not need tail anti-icing due to the fact that the horizontal stab moves over quite a large arc, due to aircraft trim requirements.

PickyPerkins
29th Nov 2004, 12:49
Milt said:Incidently FBW and artificial stability will be a way we can design for a zero tail load or even an up load on the tail for cruising flight..It's happening with military aircraft and close for non-military using artificial stability. But the elevators will then have a very busy time preventing runaway in pitch. Just curious about how an inherently unstable aircraft such as the F117 reacts to icing? Since it relies on multiple computers to fly at all, does icing make any difference (short of blocking the engine intakes and freezing the controls in place). Does it have de-icing equipment?

This web-site "http://www.grc.nasa.gov/WWW/RT1998/5000/5840ratvasky.html" describes a NASA/FAA Tailplane Icing Program (TIP), and mentions that it was initiated because of at least 16 accidents resulting in 139 fatalities attributed to such stalls. The objectives of this program were to improve understanding of iced tailplane aeroperformance and aircraft aerodynamics and to develop training aids and design tools to expand the awareness of ice-contaminated tailplane stall (ICTS). A 23-min video, "Tailplane Icing", was produced.

FAA Advisory Circular AC No: 23.143-1 "Ice Contaminated Tailplane Stall (ICTS) Date: 12/20/01" contains several warnings about the hazards of testing aircraft with stalled tailplanes, e.g. WARNING
Some hazard is associated with ICTS flight testing. It is important that the applicant take appropriate precautions in the conduct of these tests and the flight test crew very carefully considers risk mitigation that includes defining minimum altitudes, conducting tests in a build-up manner, and providing emergency escape and parachute provisions.
Cheers, http://home.infionline.net/~pickyperkins/pi.gif

alf5071h
29th Nov 2004, 16:37
Re “does seem to be a problem on some particular types”.
The mechanism of ‘tail stall’ is very dependant on the type of aircraft control system and of course the formation of ice (design and operation of the anti / de icing system).
I understand that it is rare for the tail to be fully stalled; the dominant feature in an icing 'tail stall' is a change in pitching moment as the tail force reduces – as explained above. In aircraft with manual controls, the pilot feels the change in control force on the elevator associated with the pitching moment and in extreme cases, the control column is forced forward, hence the need for a hard pull back.
In aircraft with powered controls, the pilot does not sense the actual control force, nor can the control column be moved by the surface (irreversible system).

In aircraft with effective anti icing systems (little or no ice accretion), there is little or no change in pitching moment; or where there is a small change in pitch it can be trimmed out. NB in large aircraft with trimming tails the effect is negligible and I suspect this may be a reason for not requiring anti icing systems on the tailplane.

The main risk of an icing induced tail stall (as with any icing hazard) occurs in severe icing where the anti / de icing system is overcome by the accumulating ice (definition of severe icing – aircraft type dependant). The crew are responsible for identifying and exiting the severe conditions as aircraft are not certificated for such operations.
Accidents / incidents have occurred where crew continue with an approach in unsuitable conditions (or with an icing system failure), maintain a higher than normal airspeed – often required for main wing stall protection, and then lower flap which increases susceptibility to 'tail stall'.

The modern industry is somewhat dismissive of the hazards of ice. . Please see and use ‘A Pilot's Guide to In-Flight Icing’. (http://aircrafticing.grc.nasa.gov/courses.html)

“If an aircraft is shaking it is either too fast, too slow, or covered in ice.”
“In aviation there are three types of ice. Good Ice, Bad Ice and Hazardous Ice.
Good Ice is found in the galley”.

ehwatezedoing
29th Nov 2004, 21:53
Had a Flight Safety seminar regarding icing, part of it was about tail stall.
All infos were comming from Nasa Lewis research center (as mentionned by PickyPerkins).
Now, back to my notes....

A common design lead to this kind of problems:
Turbo props fall nicelly into this category!

-Unpowered flight controls,
-Large flaps deflection,
-Deicing boots,
-Sharper horizontal tail than wings.

3 paths lead to tail stall conditions if there is ice in the tail:
-Flaps,
-Speed,
-Power.

There is usually no clue about it until configuration change.

First thing is of course correctly diagnost the problem:
You will miss those under auto pilot. And many time those are flaps full extention symptoms
-Lightening of the control, stick lightening in the forward direction,
-Difficulty trimming the A/C,
-Onset of pilot induced oscillations, (worse case scenario where the yoke will snap forward)
-Buffeting in controls not the airframe. (You must differentiate airframe buffet to yoke buffet)

To recover from a tail stall:
-Pull back on yoke,
-reduce flaps,
-reduce power. (maybe aircraft specific)

More simple--->
The universal tail stall recovering:
-Yoke back,
-Flaps to previous setting.

:ok: