Stator Vane,
The interpretation of the FARs varies according to manufacturer as I discovered this morning. A quick review of some AFM’s shows that Boeing uses 20,000 feet as a flap limit height in the 707/737/747/757/777 (I don’t have data for the 727/767). Airbus uses it with the A300. MDD and Lockheed don’t use it on the DC8/MD90/MD11/L1011.
I believe that the Boeing interpretation uses the Gust Envelope (25.341) to define a cutoff point for their testing envelope. My justification for that statement is as follows:
Positive and negative rough air gusts of 66fps at the speed Vb shall be considered at altitudes between sea level and 20,000 feet. At altitudes above 20,000 feet, the gust velocity may be reduced linearly from 66fps at 20,000 feet to 38fps at 50,000 feet.
Positive and negative rough air gusts of 50fps at the speed Vc shall be considered at altitudes between sea level and 20,000 feet. At altitudes above 20,000 feet, the gust velocity may be reduced linearly from 50fps at 20,000 feet to 25fps at 50,000 feet.
Positive and negative rough air gusts of 25fps at the speed Vd shall be considered at altitudes between sea level and 20,000 feet. At altitudes above 20,000 feet, the gust velocity may be reduced linearly from 25fps at 20,000 feet to 12.5fps at 50,000 feet.
Therefore IMHO this gust envelope combined with a desire to save flight-testing costs leads to the flap limitation.
Mutt.