How can we recover from a tailplane stall?

Question

I was studying for icing and a tailplane stall. I have looked up some internet pages and instrument flying handbook, and found the procedure below.

1. raise flaps to the previous setting. (To reduce down wash from the main wing so that reducing negative angle of attack of the tail and break the stall)

2. apply nose up elevator pressure (I don't get it. The nose up pressure will make the elevator to go up and wouldn't this increase the negative angle of attack and worsen the stall?)

3. do not increase airspeed unless it is necessary to avoid a wing stall. (Why shouldn't we increase airspeed?)

So now I'm trying to understand the reason why should a pilot do such actions. Can you help me out?

Answer 1

Lets start with the very basic concepts....

In most aircraft, the Centre of Gravity (cg) is somewhat forward of the wing or mainplane Centre of Pressure. The exact distance between the cg and the Centre of pressure will depend on aircraft loading, configuration, thrust setting and drag. However, cg forward of the Centre of Pressure produces a nose-down pitching moment. The horizontal stabilizer, or tailplane, then provides a downward force to overcome this normal, nose-down, pitching moment. The tailplane behaves as an ‘upside down’ wing and operates with negative Angle of Attack (AOA) as shown in Figure 1

Positive and Negative Angle of Attack

Figure 1 - Positive and Negative Angle of Attack

If the horizontal stabiliser becomes contaminated with ice, airflow separation from the surface can prevent it from providing sufficient downward force or negative lift to balance the aircraft and a nose-down pitch upset can occur.

When compared to an aircraft's mainplane, the horizontal stabiliser normally has a thinner aerofoil with a sharper leading edge. Differences in the ice collection efficiency or catch rate between the two surfaces means ice accumulates faster on the horizontal stabiliser and may form before any ice is present on the aircraft's mainplane.

Tailplane stall can occur at relatively high speeds, well above the normal 1G stall speed of the mainplane. Typically, tailplane stall induced by icing is most likely to occur near the flap limit speed when the flaps are extended to the landing position, especially when extension is combined with a nose down pitching manoeuvre, airspeed change, power change or flight through turbulence. Aircraft stall warning systems provide warnings based on an uncontaminated mainplane stall so during a tailplane stall induced upset there will be NO artificial stall warning indications, such as a stick shaker, warning horn or the mainplane or flap buffeting normally associated with a mainplane stall.

Tailplane Stall Aerodynamics

1. The horizontal stabiliser, or tailplane, of an aircraft is an aerofoil that provides a downward force to overcome the aircraft's normal nose-down pitching moment. The further forward the Centre of Gravity is from the Center of Pressure, the greater the nose down moment and, thus, the greater the amount of down-force that must be generated by the tailplane. This, in turn, requires a greater negative tailplane angle of attack (AOA). angle of attack (AOA). [As shown in Figure 1, The tailplane is effectively an upside down aerofoil so an increase in negative tailplane AOA occurs with UP elevator movement or when the aircraft is pitching nose down.]

2. Accumulation of ice on the tailplane will result in disruption of the normal airflow around that surface and will reduce the critical (or stalling) negative AOA of the horizontal stabiliser.

3. Ice can accumulate on the tailplane before it begins to accumulate on the mainplane or other parts of the aircraft.

4. Flaps extension usually moves the mainplane Centre of Pressure aft, lengthening the arm between the Centre of Pressure and the cg and increasing the mainplane nose down moment. More down force is required from the tailplane to counter this moment, necessitating a higher negative tailplane AOA.

5. Flap extension, especially near the maximum extension speed, increases the negative tailplane AOA due to the increase in downwash, as shown in Figure 2

6. Increasing the power setting on a propeller driven aircraft may, depending on aircraft configuration and flap settings, increase the downwash and negative tailplane AOA.

7. When the critical negative AOA of the horizontal stabiliser is exceeded causing it to stall.

8. Tailplane stall drastically reduces the downward force it produces, creating a rapid aircraft nose-down pitching moment.

Effect of mainplane flap on downwash

Figure 2 - Effect of mainplane flap on downwash

On aircraft with reversible (unpowered) elevator, tailplane airflow changes caused by ice accretion may lead to an aerodynamic overbalance driving the elevator trailing edge down and pitching the aircraft nose down. This can occur separately from or in combination with the nose down pitching moment caused by tailplane stall. The yoke may be snatched forward out of the pilot’s hands and the control force required for the pilot to return the elevator to neutral or to a nose-up deflection can be significant and potentially greater than the pilot can exert.

now match with your recovery actions

1. You have no doubt with your 1st point.
2. The second point: You have to resist the nose down elevator movement. Once the tailplane is already stalled then you have to assume the elevator has already gone in down position to make your ac nose down. So you have to apply nose up elevator pressure.
3. You should not increase the airspeed cause it might make the situation worse. Cause in dive with increased airspeed is always difficult to maintain the aircraft control. And you might end up with overstressing the elevator which is not good at all in such condition.

Answer 2

There was a NASA report on the NASA/FAA Tailplane Icing Program Overview, which covers the points raised by you. It lists certain actions that can be done to recover from a tail plane stall:

When the full tail stall was experienced during the power transition, the stall recovery procedure was:

• Reduce thrust (may be airplane specific)

• Pull back on yoke/ increase $\alpha$

• Raise flaps

• Raise flaps to previous setting

  This is done mainly to undo the changes that caused the stall in the first place (reducing thrust is also done for this reason). The report says,

The major lesson learned to recover from a tail stall was to undo what was just done to cause the event.

• Apply nose up elevator pressure

  Basically you are pulling back on the yoke to increase the tail download so that the aircraft nose-down pitching moment is countered. From the report:

Pulling back on the yoke increased the camber of the tailplane, which provided enough tail download to counteract the nose-down pitching moment and increase the $\alpha_{tail}$

This sounds counterintutive as the conventional tail produces force in the direction opposite to the main wing. The report notes:

It was noted that this tail stall recovery procedure is opposite of the recovery from a wing stall. The reason for the difference is the location of the flow separation. In a wing stall, the flow separates from the upper surface of the wing, therefore reattachment is made by decreasing the wing $\alpha$. In a tail stall event, the flow separates from the lower surface of the tail and requires a positive increase in tail to reattach the flow.

• Do not increase airspeed unless it is necessary to avoid a wing stall

  The aircraft is already at a nose down attitude. Increasing speed will further excabarate the situation, which may put the corrective action beyond the capability of the tailplane.

Answer 3

For those recommended actions to be effective, two preconditions have been quietly assumed:

1. The tail surface produces downward lift and
2. The wing has positive camber.

Both can be assumed to be correct in almost any case. Now let’s look at the three recommendations in detail:

raise flaps to the previous setting.

Flaps increase camber and shift the center of pressure backwards. In order to balance the aircraft with the same center of gravity location, the tail needs to produce more downward force with lowered flaps. Retracting flaps will unload the horizontal tail and reduce the stall condition.

apply nose up elevator pressure.

This adds tail camber and helps to produce the same downward force at a less negative local angle of attack of the stabilizer. The induced angle of attack of the added tail camber will increase the local angle of attack at the stabilizer. This can only help momentarily, though, because it will make the aircraft pitch up and lose speed - unless you have a movable stabilizer which is used for trim. Re-trim with the new elevator setting and the change becomes permanent.

do not increase airspeed unless it is necessary to avoid a wing stall.

When the main wing has positive camber, a lower wing angle of attack shifts its center of pressure backwards. Therefore, the tail load and lift coefficient are lowest at low speed, and flying slowly will unload the tail. With rear center of gravity, tail load normally can even become slightly positive at low speed.

So in all cases the recommendations help to unload the tail and reduce the condition that let the iced tail stall.

Answer 4

I have a theory on applying the backstick on a tail stall induced while lowering the flaps. OP’s comment:

apply nose up elevator pressure (I don't get it. The nose up pressure will make the elevator to go up and wouldn't this increase the negative angle of attack and worsen the stall?)

If the tail stalls, then the nose pitches forward. The center of rotation is about somewhere near the wing and CG, well forward of the tail. So the tail instantly sees a step increase in AOA, to well beyond the stall AOA. Probably really high AOA. The aircraft develops inertia in pitch too.

At this point, you are doing triage while you lower the flaps and undo what you did. While you’re not getting much lift (downward) from the tail, it is resisting the forward pitch with drag. Letting the stick or yoke come forward would dump this pitch resisting force, and the aircraft would go right over its nose before the flaps could raise.

There’s my theory.