ATPL question on the effect of weight on cruise level and airspeed
Thread Starter
Joined: Dec 2006
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From: aussie land
ATPL question on the effect of weight on cruise level and airspeed
Hi, my ATPL text is a little bit confusing on this topic so I hope someone here can help to clear things up. 
So far, I've learnt that max range is obtained at 1.32 Vimd and that the angle of attack where this is achieved is constant. So when fuel is burn off during cruise and the aircraft becomes lighter, in order to fly at 1.32 Vimd (and at constant angle of attack), the aircraft will need to fly slower at the same altitude. BUT, as an alternative to flying slower, a lighter aircraft can choose to climb and fly at a higher altitude.
So, my question is, when it chooses to do this, ie, climb to a higher altitude as a result of fuel burn off and becoming lighter, is this aircraft still flying at 1.32 Vimd and at the same IAS as it was before the climb?
So far, I've learnt that max range is obtained at 1.32 Vimd and that the angle of attack where this is achieved is constant. So when fuel is burn off during cruise and the aircraft becomes lighter, in order to fly at 1.32 Vimd (and at constant angle of attack), the aircraft will need to fly slower at the same altitude. BUT, as an alternative to flying slower, a lighter aircraft can choose to climb and fly at a higher altitude.
So, my question is, when it chooses to do this, ie, climb to a higher altitude as a result of fuel burn off and becoming lighter, is this aircraft still flying at 1.32 Vimd and at the same IAS as it was before the climb?
Nemo Me Impune Lacessit
Joined: Jun 2004
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From: Chabanais, France
Given that under most circumstances IAS will decrease with altitude if you maintain the same Mach # then no, if you maintain V1.32 Vimd then, at a greater altitude, subject to temperature, the IAS will be lower.
Use the example that the sound barrier will be broken at sea level at around 639knots, IAS, subject to temps and pressure but at altitude the barrier will be broken at a much less IAS.
Use the example that the sound barrier will be broken at sea level at around 639knots, IAS, subject to temps and pressure but at altitude the barrier will be broken at a much less IAS.
Joined: Feb 2007
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From: -------
Parabellum, you're confusing JulieFlyGal much more then he/she already was. If you read carefully the post, Mno is never mentioned, because in this situation Mno is not involved at all.
Vimd is a KIAS speed (all V-labeled speed are IAS/CAS/RAS/EAS).
For a lighter weight the Vimd is different, because the total drag curve is different (less weight, same parasite drag but different induced drag, the curve goes slightly to the left and Vimd a bit lower) and so Max range speed is also lower.AOA constant.
FB
Vimd is a KIAS speed (all V-labeled speed are IAS/CAS/RAS/EAS).
For a lighter weight the Vimd is different, because the total drag curve is different (less weight, same parasite drag but different induced drag, the curve goes slightly to the left and Vimd a bit lower) and so Max range speed is also lower.AOA constant.
FB
