As pilots, most of us are familiar with structural icing and the dangerous, sometimes fatal, situations it can cause. But did you know that icing on your horizontal stabilizer can result in a tailplane stall that requires opposite stall recovery techniques?

Aircraft That Are Affected

A tailplane is another word for your horizontal stabilizer. Aircraft that use unpowered controls (those that use aerodynamic balance) to keep stick controls neutral are most susceptible to tailplane icing. In general, this applies to aircraft with a fixed leading edge horizontal stabilizer, where the elevator moves and is held neutral during flight by elevator trim tabs.

Tailplane stalls result from ice accumulation on the leading edge of the horizontal stabilizer. So airplanes with no de-icing or anti-icing systems that fly into icing conditions are most at risk. And even if you have a known-ice equipped aircraft, if your equipment fails or isn't used properly, you'll be just as susceptible. Throughout the article, remember that tailplane icing stalls are uncommon and most frequently seen on mid-sized turbo-prop aircraft flying through icing conditions.

When Tailplane Icing Occurs

As you probably know, in straight and level flight, the vertical forces acting on an aircraft are weight (downward), the wing's lift (upward), and the tail's component of lift (downward, which is also called "tail-down force"). Since the center of gravity is almost always forward of the aircraft's center of lift, a downward pitching moment is created which must be counteracted by tail-down force. And to achieve this downward lift, the tailplane is designed like an upside-down wing.

During cruise, ice isn't as big of a concern on the tail as it is on the wings. This is because the the tail is usually at a low angle of attack and nowhere near performance limits. Any separated airflow around the tailplane stays relatively close to the ice buildup, allowing a majority of the effective airflow to remain attached around the tailplane and elevator. This changes and the greatest risk of a tailplane stall occurs when you increase flaps, or sometimes, power.

A few things happen as you add flaps:

1) The wing's center of lift moves aft, creating a large pitching down moment that the horizontal stabilizer must counteract.

2) The tail's angle of attack increases due to the increased wing downwash.

3) More downward lift may be required as the aircraft reaches equilibrium. This is accomplished by pulling back on the yoke and moving the elevator's trailing edge upward. Consequently the flap extension drives the horizontal stabilizer towards its critical angle of attack.

This is when tailplane icing becomes a serious problem. Even a small amount of icing will interfere with airflow around the lower surface of the horizontal stabilizer, resulting in airflow separation. So as angle of attack increases, the re-attachment point for separated airflow moves aft. If this new re-attachment point extends over enough of the elevator, beyond the hinge point, the moveable elevator control surface will fill the airflow void and move downward, causing the airplane to pitch downward.

In short, large flap deflections, which produce large amounts of downwash, result in high angles of attack on the tailplane. With any ice accumulation on the leading edge of the horizontal stabilizer, increasing flaps, and sometimes power, will increase airflow separation around the tailplane. This is when you're most at risk for a tailplane stall.

Warning Signs

Although they differ based on aircraft and airfoil type, here are some of the common warning signs that NASA discovered when it comes to tailplane stalls:

1. -Lightening of the controls (stick feels light in the forward direction)
2. -Difficulty trimming the airplane
3. -Onset of pilot induced oscillations
4. -Buffeting in the controls, not the airframe

Most of these symptoms were noted with flaps at full deflection. In extreme conditions, a rapid pitch down control pulse can be felt and recovery may be impossible on final approach due to low-altitude. Also, if you're flying on autopilot, you'll probably miss many of these signs because you won't get any tactile feedback from the controls.

Wing Stall vs. Tailplane Stall

In a conventional wing stall, the airplane pitches down due to a loss of lift. A tailplane stall also results in pitching down, but because tail-down force has been lost. This is why stall recovery techniques are opposite. Aircraft flight manuals can differ, but here are NASA's general guidelines to recovery:

Wing Stall Recovery:

1. -Add Power
2. -Relax back pressure or push yoke forward

Tailplane Stall Recovery:

1. -Pull back on the yoke
2. -Reduce Flaps
3. -Reduce Power: This is aircraft specific based on engine location in relation to CG and how power changes angle of attack. (Engines mounted above the CG will create a stronger pitching down moment as power increases)

Identifying The Stall

Identifying a wing stall vs. a tailplane stall isn't easy since the differences in warning signs are subtle. You need to be able to differentiate airframe buffet from control buffet. With airframe buffet, you will get feedback through the seat of your pants, as opposed to just the buffeting of the controls in a tailplane stall. The greatest risk pilots face is misinterpreting a stall incorrectly, be it a wing stall or rare tailplane stall.

Aircraft configuration and speed is another way to identify the stall correctly. If flaps are lowered at the higher end of flap extension speed limits and there is an elevator control buffet, chances are there is a tailplane icing problem. The higher the speed with flaps extended, the more susceptible the aircraft is to a tailplane stall.

What You Can Do

If the aircraft is equipped with a pneumatic de-icing system, it goes without saying that you'll want to activate the system several times to try to clear the ice off of your horizontal stabilizer. If you still experience problems, you might have to land with reduced flaps. And if you're airplane isn't equipped with a de-icing system, exit icing conditions as quickly as possible and make a judgement on how quickly you need to land. Just remember how you were trained to deal with icing encounters and be aware that changing aircraft configuration could put you at greater risk of a tailplane stall.

Easy enough, right? If the leading edge of your horizontal stabilizer accumulates ice and airflow separation increases around the tailplane due to an increased angle of attack, you're at great risk for a tailplane stall. And while tailplane stalls may be uncommon, knowing the warning signs and recovery techniques could save you from a potentially devastating accident.