Low-Speed Loss of Control

Understanding angle of attack is the key.

From an NTSB preliminary report on an accident involving a Piper PA-46-500TP (Meridian turboprop single): "An eyewitness, a retired Navy instructor pilot, located about one half-mile northwest of the accident site, reported observing the accident airplane descending through the overcast about 1227. The airplane leveled off about 300 feet above ground level (agl) and turned south on a compass heading of approximately 195 degrees. A couple of minutes later the witness observed the airplane heading north on an approximate heading of 15 degrees but at a 'much slower' airspeed. The eyewitness then observed the airplane roll right to an approximately 60-degree angle before the nose of the airplane fell through to an 'extreme nose-low attitude.' After the airplane disappeared behind trees, the witness heard a crash and observed black smoke. The witness further stated, 'The [airplane] exhibited a classic approach turn stall maneuver I had taught many times, but this time with no altitude to recover.' "

The pilot was trying to fly an ILS approach and the accident occurred after the pilot reported he was having trouble performing a "coupled" approach and that he was trying to "get control" of the airplane.

A lot of dominoes tumbled there, but in the end it apparently became what is referred to as a stall/spin accident. We often think of such accidents as typically involving lower-powered airplanes maneuvering at low altitude, and a lot do, but a surprising number of them, about half, occur in airplanes used for transportation. There is usually a distraction, as there was in this example, and in the end the pilot simply doesn't allow the airplane to fly.

There were a couple of other accidents in the recent past that relate to this. One was a turboprop, one a turbojet. One had an owner-pilot and an experienced keeper-pilot; the other had two professional pilots. In both cases the airplane was apparently leveled off after a descent but no power was added, and a low-speed loss of control was the result.

In the all-time classic book on flying technique, Stick and Rudder, Wolfgang Langewiesche devoted the first 24 pages to a discussion of the importance of understanding angle of attack. He didn't think most pilots understood much about the subject. Do they now?

Years ago, in the early '50s, the FAA got caught up in this and launched an educational program on angle of attack for flight instructors. This is from a distant memory, but as I recall they equipped one of the first PA-18s with angle of attack instrumentation that measured five different angles. Lights in the cockpit would show when each angle was reached and the drill was to honk the airplane around and watch the angle of attack change. Get four lights on and it was close to the stall; five lights and it let go.

Also in the early '50s, Leonard Greene of Safe Flight Instrument Company developed what he initially called the "Landing Speed Indicator." It was basically an angle of attack indicator that showed, with a centered vertical needle, when the wing was at the angle of attack for maximum lift. This was later marketed as the Safe Flight Speed Control System, $345 for a basic model, and I had one in my Piper Pacer. This did not catch on as a product, though Safe Flight is still very much in the business of developing and manufacturing sophisticated systems for turbine airplanes.

Pilots must feel that they can do just fine using the airspeed indicator, and they can, but an awareness of angle of attack becomes critical when there are distractions or when the chips are down. A pilot who levels off but doesn't advance the power has to realize that the angle of attack will increase to the point of a stall if the airplane is flown level without power. That is terribly basic but it is sometimes neglected with bad results.

I don't know how widely understood angle of attack is today, so bear with some basics for just a second. I used to describe it to students as the difference between where the nose is pointed and where the airplane is going. I would demonstrate it in slow flight, with the nose up, say, 10 degrees with the altimeter steady and/or the vertical speed on zero. The nose is up 10 degrees, the airplane is not going up, so the angle of attack is approximately 10 degrees.

Most wings stall at an angle of attack from 15 to 18 degrees. The exact angle doesn't really matter. What does matter is for the pilot to understand what might be about to happen when the angle of attack is nearing that critical value.

I always think back to the light system in the Cub and to Doc Greene's speed control system. To manage angle of attack properly, you have to think like those devices. Two lights on or the needle in the middle is just right. Anything on the slow side of that is b-a-d bad. I always wondered why the stall warning indication didn't start when the angle of attack went beyond that for best lift and increase in volume as the stalling angle got closer. Without such, we just have to think that way. We have to think about the airplane being on the bad side of good.

Airplanes communicate with us in different ways. I know that a lot of pilots don't agree with this, but I have always thought that trim is overused in some phases of flight. It is always best to trim an airplane for a safe airspeed, be it during climb, in level flight or on approach. What I don't agree with is masking an increase in angle of attack with trim. I have, for example, never trimmed in steep turns or trimmed beyond approach speed during landing. That's because the out of trim condition lets the airplane talk to me about what is going on. If I am pulling back on the wheel or stick, I am increasing the angle of attack.

Because most low-speed losses of control come during low-altitude maneuvering flight, nothing that we learn practicing stalls does any good when the chips are down. When control is lost, there is simply not enough altitude for that conventional stall recovery that is practiced over and over. The technique we have to learn and practice is angle of attack management.

This can be done with the airspeed indicator, using minimum acceptable safe airspeeds. One point three times the stalling speed for the aircraft weight and configuration is the standard for determining a reference speed for a normal approach. To that has to be added some for the increase in stalling speed with bank. That gives two speeds, one for wings level, one for a bank-up to a maximum of 30-degrees with each speed applied to various flap settings.

For example, on a Cessna 400 the flaps-up stalling speed is 69 knots indicated airspeed, so a minimum flaps-up speed would be 90 knots, wings level. In a 30-degree bank that would increase to 95. In a 45-degree bank it would go up to 105, and that rather dramatic increase is the primary reason that wise pilots never exceed 30-degrees of bank in low altitude maneuvering flight. More bank moves the angle of attack closer to the critical value. For takeoff (half) flaps, all stalling speeds are reduced by five knots; landing (full) flaps takes off another five knots. All speeds would be lower at lighter weight, reduced by half the percentage the airplane is below the maximum weight of 3,600 pounds. Any time the airplane is flying at speeds lower than those calculated values, trouble is near. An angle of attack indicator would calculate all of that for you.

Because low-speed loss of control accidents almost always begin at low altitude, we have to consider the properties of the atmosphere when maneuvering and when the altitude is changing. There is almost always at least a little bit of wind shear at low altitude, and an angle of attack indicating device will help a pilot fly through this without letting the airspeed decay below the calculated minimum value. Or, we can imagine what is coming and greet it with at least some anticipation.

Wind shear will also show on the airspeed indicator, with a decreasing headwind resulting in a decay of airspeed. This is a likely scenario when we are gliding into the wind and the wind velocity is less close to the surface than it is a few hundred feet high. That would mean the nose of the airplane would have to be lowered to maintain airspeed even though that wouldn't look good.

I am going far back for an example here, back to the Ercoupe that was built mostly in the '40s. Because it was stall resistant and spin proof it was touted to be a "safe" airplane. As happens to most airplanes that pilots consider to be "safe," the Ercoupe had an abysmal safety record when compared with other two-place airplanes of the day.

The bad Ercoupe record had more to do with pilots thinking it was "safe" than it did with the airplane itself. And, because the effects of wind shear were not widely acknowledged at the time by the pilot population, many were puzzled by the fact that the Ercoupe had a lot of what looked like stall-mush type accidents. How was that possible in a stall-resistant airplane?

Simple. Pilots thought nothing of flying Ercoupes very slowly. When they would, say during a forced landing, fly from a headwind condition to a no-wind condition, the airplane would pitch nose down to try to maintain airspeed. And it would hit the ground nose down, just as if it had stalled. It couldn't stall because of restricted up-elevator travel but it could sure hit the ground hard and nose down.

So, if we keep the airplane trimmed for a safe airspeed, limit bank angles to 30 degrees when below a couple of thousand feet, and are aware that any time we add up-elevator pressure we are moving the angle of attack from a good place to a place closer to the stall, that should take care of things.

It doesn't always, though, as witness the fact that about a quarter of the serious injury/fatal accidents in general aviation are related to a low-speed and low-altitude loss of control.

The number one trouble spot is low-altitude maneuvering. This might be done for any number of reasons and is sometimes related to buzz jobs, or to what the NTSB sometimes calls an "ostentatious display." Whatever, it is a relatively easy thing to stay away from.

Next on this list is maneuvering for an approach to a runway. This one is pretty simple, too. Mind the airspeed and if a bank of more than 30 degrees is required to line up, abandon that approach and make a better plan.

The initial climb after takeoff also figures in a number of these accidents. Short fields are often involved. Just remember that right after liftoff and in initial climb the angle of attack is relatively high. The minimum airspeed here would be 1.2 Vso or the best angle of climb speed in the POH. If obstacles can't be cleared at that speed, they can't be cleared and a controlled crash is the safest option. A reason for that in a moment.

Forced landings are a major factor and here it is just a matter of keeping the airplane flying until it reaches the ground or hits something.

Go-arounds usually start with the drag high (because of extended flaps) and the angle of attack high. They are something to practice at altitude.

There are a number of low-speed losses of control in twins on the list. These come after one engine fails and the pilot fails to maintain airspeed. With asymmetic thrust, light twins are positively lethal if the airspeed is not minded. It is amazing how many pilots don't get their money's worth out of having a second engine simply because they don't care enough to maintain the required proficiency.

Low-speed loss of control accidents are always serious. This is true even of airplanes with quite low stalling speeds. I ran across an old but valid study on this done by Dr. E. Jeff Justis and printed in the Legal Eagle News. He said, " ... a stall/spin accident would generally be considered to have high vertical loading. On the other hand, ploughing into trees or a fence or other object would be an accident in which horizontal loading would predominate."

Jeff then looked at the number of hospital days required to recover from injuries suffered in an accident with predominately vertical deceleration as compared with horizontal. Vertical took 44.6 days; horizontal 15.7 days. So, avoiding a low-speed loss of control is definitely good for your health.