EAA Sport Aviation - April 2012

condition per the practical test stan-
dards is the reason we as pilots place so
much emphasis on the airspeed num-
ber equating to a stall.

Don’t get me wrong, using the V speeds as a reference is just fine. My point is there is much more to avoiding a stalled wing than simply avoiding slowing below a single speed point on the airspeed indicator. It is more important to visualize what is truly happening with the AOA of the wing in every flight condition. Change the bank angle or cause an abrupt pitch change (acceleration) and you might as well throw that V speed reference out the window.

One of the first things you can do to learn more about AOA is do an Internet search for some of the aviation crash videos on You Tube. Unfortunately, there are plenty to choose from that clearly show a low-altitude accelerated stall and impending crash. Keep in mind what is usually referred to as a low-altitude stall/spin is actually a stall and wing drop resulting in a loss of control. The airplane rarely appears to be spinning before striking terra firma. Note the seemingly normal appearance

A bad landing is almost always the result of a bad approach. Go-arounds are your friend!

of the flight path right up to the point of losing control. That tells you the airplane was in an accelerated state and exceeded the critical AOA at a higher than VS stall speed. The pilot abruptly pulled, the wing stalled, and the result was a crash caused by the lack of altitude to make a recovery.

One of the best demonstrations I have
seen of an accelerated stall is in the North
American T- 6 military trainer. Rolling the
airplane over into a steep bank, allowing
the airspeed to build a bit in a descent,
and then rapidly increasing the AOA
(loading some g’s on the wing) produces
the most pronounced buffet you can
imagine. It is as if there are a dozen mon-
keys all stomping on the wing as the
trailing half of the wing loses laminar
flow and the air becomes turbulent. The
wing is not fully stalled, mind you, but it
is definitely past CLMAX. This all is occur-
ring in a condition that results in a
significantly higher airspeed indication
than the published VS. Pull just a bit more
and the airplane will “unhook” com-
pletely and enter a fairly brisk roll, thus
showing you why it has hurt people
attempting low-level higher g maneuvers
at air shows without the proper under-
standing and respect of AOA.

Sean Elliott, EAA 520258, is EAA vice president of advocacy and safety.

FAA Clears Path for AOA Systems

The FAA recently ruled that installing an angle of attack (AOA) measuring system in a non-pressurized piston airplane is a minor modification, meaning no STC or FAA field approval is required. An aircraft logbook entry by an A&P who performed the work is all that is necessary after installation. The AOA system cannot—and really doesn’t—replace any airspeed indication or other systems in the airplane.

The new policy is aimed initially at AOA equipment from Alpha Systems, which has been making sensors for many years but lacked the STCs and other approvals to install the equipment in most certified airplanes. Since the FAA is emphasizing AOA awareness as a means to avoid stall-spin accidents, it makes sense to encourage airplane owners to install AOA sensors with the least cost and hassle possible. The new policy allows that to happen.

The Alpha Systems sensor uses two air ports mounted on a short mast that is typically mounted to the lower surface of the wing to measure AOA as the slipstream flows over the ports. The lowest cost systems that use only pneumatic pressure from the ports costs around $800. Alpha Systems also offers more advanced electrical displays that use lights and graphics to show pilots AOA, and those systems can range in cost up to around $2,000.

The new FAA policy will also apply to other companies that make AOA sensing equipment, such as Safe Flight Instruments, which has manufactured hundreds of thousands of stall warning systems.