Alpha Vains

[jgibbs]

Does anyone know exactly how alpha vains work on the 737ng??
Also when switching the apu from off to start on the 737 ng, what triggers the switch to goto the on position? Is is springloaded like the bus panel switches or does it switch back to on during a certain point in the start process (ie ignition or a certain RPM value)?
Thanks
Jon Gibbs

[QAVION]

"Does anyone know exactly how alpha vains work on the 737ng??"

I guess you're referring to the Angle Of Attack Sensor(s) on the side of the airplane (not far from the nose)?

I'm pretty sure these would work the same way they do on other aircraft. They are simply small wing-shaped or wedge-shaped devices which pivot up and down when the airflow around the fuselage is fast/strong enough to move them. They sense the angle of the fuselage with respect to the airstream/flow in which the aircraft is travelling.

There is an electrical transformer (hidden) inside the aircraft which senses the angle of each vane (wing/wedge). This transformer converts the angle into an electrical signal which is then sent to the aircraft's air data computers. This AOA data is can be used for a variety of things (e.g. to help generate stall warnings).

Here's a shot of a 747-400 AOA sensor:

http://www.ozemail.com.au/~iriddell/AOASensorNew2.JPG

Please ignore the red line (it was added later to show the "neutral position")

"Also when switching the apu from off to start on the 737 ng, what triggers the switch to goto the on position? "

It's the same as an Airbus. It simply stays in the Start position as long as you hold it there. Otherwise is simply springs back to ON.

Rgds.
Q.

[fantom]

now are these vains; vanes or veins?

:confused:

[Captain Stable]

and what happens to the airflow if the aircraft has varicose vanes? ;)

[northwing]

Not familiar with 737, but some alpha sensors look at first sight like a metal cylinder sticking out of the side of the fuselage, about 2 inches long and a quarter inch diameter. They have no wing-like surfaces on them and it is not immediately obvious how they align themselves with the flow. In fact they have a slit along one side. This connects to a circular chamber inside the fuselage skin which has a vane in it and the external cylinder moves until pressure is equalised either side of this vane. Easier to de-ice, less drag, less vulnerable to birdstrikes than an external vane. These vanes always over-read because of the local flow round the fuselage, but this is normally taken out in converting the signal to the display so that the alpha gauge normally reads in units that are something like degrees of incidence. It is normal to have a sensor each side of the fuselage to take out the effects of sideslip.