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Trying to understand

Asked by: 3729 views Aerodynamics

Below is a paragraph I read in a aviation book. I am interested to understand this paragraph further.

Maneuvering speed is 145 knots for airplane ABC. At 25,000 feet true airspeed is 200 knots and 134 knots indicated of which is 11 knots below maneuvering speed of 145 knots. During a turbulence encounter, the aircraft would not have the stall protection it has a lower altitudes. Above 25,000 feet, indicated airspeed and the available stall margin become significantly less. The obvious solution is to descend.

Maneuvering speed is 145 knots for Cessna 414. At 25,000 feet true airspeed is 200 knots and 134 knots indicated of which is 11 knots below maneuvering speed of 145 knots.
During a turbulence encounter, the aircraft would not have the stall protection it has a lower altitudes. (I assume author is implying since the air is very thin/less dense at/above 25,000 feet the airplane must be flown at a higher AOA to maintain acceptable lift of which does result in being closer to the critical AOA). Please explain further??
Above 25,000 feet, indicated airspeed and the available stall margin become significantly less. (I assume author is implying since the air is very thin/less dense at/above 25,000 feet the airplane must be flown at a higher AOA to maintain acceptable lift of which does result in being closer to the critical AOA). Please explain further??
The obvious solution is to descend. (I assume author is implying that at lower altitudes since the air is denser that the AOA to maintain acceptable lift is lower). Please explain further??

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2 Answers



  1. Dan S. on Aug 16, 2015

    It sounds like the author is addressing the issue that at the maneuvering speed of 145 knots, the aircraft would be protected from damage in the case of severe turbulence, while enjoying a wide berth between the stall speed and this speed. But since the calibrated airspeed at higher altitudes is going to be less for the same TAS that would be accomplished at lower altitudes, you start to run into controllability issues as well as a smaller margin between your calibrated airspeed and the stall speed because of air density. Remembering that significant load factors (that may be encountered in turbulence) can cause a sudden increase in stall speed while the load factor is increased. Essentially, you are travelling much faster through the air at higher altitude, but the aircraft acts as if it were travelling at a slower airspeed because of decreased air density. And this would be indicated on your airspeed indicator. This puts you closer to stall speed, so less margin of safety. By descending to a lower altitude, this margin would be increased, and the safe decision would be to descend in the event of strong turbulence at high altitude (And believe me, you get some rough stuff in the higher altitudes on occasion). Check out AC 61-107B when you get a chance.

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  2. Dan Chitty on Aug 25, 2015

    Thank you for the feedback.

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