Which of the following factors could cause an erroneous airspeed indication?
Air Speed Indicator is the only instrument that uses both Static and Dynamic pressure. It shows Indicated or Calibrated speed, and with a mechanism that compensates for altitude and temperature corrections, it can also show True Air Speed. It has Arcs and Markings for a big variety of speeds like, Vs, Vso, V flaps extend, Vno, Vne.
The ASI uses a dynamic capsule that receives TOTAL pressure from the Pitot tube. If we don’t remove the Static pressure from inside the capsule, then we are going to have a false indication of greater speed. The only way to eliminate the Static pressure from inside the capsule, is to feed the case of the instrument with Static pressure. That can be easily done, if we connect the case with the Static source.
Since the Static pressure from the case is the same with the Static pressure inside the capsule, one eliminates the other and we finally have Dynamic pressure inside the capsule, that is translated to SPEED.
Suppose aeroplane climb and the Static port becomes blocked. Then, after a while, the Static pressure inside the capsule will be way more than the blocked Static pressure inside the case. That will force the capsule to expand more, showing more speed than the actual, creating the danger to stall the aircraft.
The opposite will happen in a descent, and the danger is to overspeed the aircraft because the actual speed will be lower than the indicated.
In case, now, that the Pitot tube becomes completely blocked, then the ASI will react like an altimeter.
An erroneous airspeed indication could be caused if the aircraft is covered with adhesive foils.
Your Notes (not visible to others)
This question has appeared on the real examination, you can find the related countries below.
-
Austro Control