Tuesday, 23 May 2017
MANAGING ANGLE OF ATTACK
Acknowledgements: AIR FACTS/Richard Collins
Richard offers the following thoughts on the subject of AoA:
“Managing angle of attack (AOA) is a critical element in flying safely but it often isn’t emphasized enough in training. Make no mistake though, it is real and a pilot who understands only airspeed control is lacking in basic knowledge.
When I was instructing I put a lot of emphasis on students understanding angle of attack. This was critical in the lower powered airplanes with not a lot of margin between cruising and stalling speed.
I told my students that the angle of attack in level flight was the difference between where the nose is pointed and where the airplane is going. Power back and slow down and the nose might be high but the altimeter steady or showing a decline. Push that difference between where it is pointed and where it is going up to about 18 degrees and the airplane would stall.
The plot thickens in level turning flight, where the relative weight increase because of G-loading results in an increase in angle of attack. Most pilots are aware that stalling speed increases a bit in a 30-degree bank and a lot when the bank becomes even steeper as the g-load in a level turn increases, but I tried to put across that while the stalling speed might vary based on conditions, the stalling angle of attack is a constant.
In virtually all low speed loss of control accidents, the pilot was pulling back on the elevator control when control was lost. Back stick increases angle of attack. A pilot should not need instrumentation to tell him when angle of attack is encroaching on the stalling point, but maybe the number of this type of accident suggests that pilots do need help, even though this should be relatively easy for a pilot to visualize.
I actually had an angle of attack indicator in my Piper Pacer in the 1950s. It showed angle of attack on an instrument. It had a tick mark, and when the dial was on that the angle of attack was presumed right at the value for maximum lift. If the dial was to the right the angle of attack was less, to the left it was more. Max. performance climbs and short field approaches were flown with the needle on the tick mark.
All airplanes do have a form of AOA instrumentation in the stall warning indicator, which simply tells you that the AOA is nearing the stalling point. The stall warning is supposed to sound five knots above the stall, even though the stall warning itself is based on angle of attack and not airspeed.
So, would angle of attack instrumentation help to reduce the number of stall/spin accidents? I don’t know, but the fact that most of the airplanes in these accidents have stall warning systems suggests otherwise. It seems to me to be more of an area for education.
Also, the criteria for the stall warning to sound might need to be revised upward and relate to angle of attack instead of airspeed”.