power question
05-12-2009, 10:08 AM
#21
Gets Weekends Off
Joined APC: May 2009
Position: Another RJ FO
Posts: 1,272
People in this thread are missing the main point of the power reduction upon reaching cruise altitude: Money. Planes operate at best fuel economy power settings which can be quite a bit under maximum climb power. You even see this power reduction in small planes when setting power for best economy. Normally if a plane climbs at 2700 RPM, best economy will have mixture leaned to peak EGT, and power set around 2300 RPM depending on density altitude. The airlines want to make money. The extra fuel required to get you to your destination 30 minutes sooner is not cost effective.
05-18-2009, 08:56 PM
#22
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Joined APC: Dec 2007
Posts: 1,068
EX/On SWA yesterday, climb N1 was roughly 99% at top of climb. Reduced to 84% to maintain mach .75.
With many large aircraft, the FMS takes into account the wind, temp, cost index etc. and provides the crew with a rough power setting to maintain desired mach/IAS in both climb and cruise. In many aircraft, the auto throttles will maintain a selected mach number.
05-25-2009, 08:18 PM
#23
Gets Weekends Off
Joined APC: Mar 2008
Position: 737 Capt
Posts: 1,166
People in this thread are missing the main point of the power reduction upon reaching cruise altitude: Money. Planes operate at best fuel economy power settings which can be quite a bit under maximum climb power. You even see this power reduction in small planes when setting power for best economy. Normally if a plane climbs at 2700 RPM, best economy will have mixture leaned to peak EGT, and power set around 2300 RPM depending on density altitude. The airlines want to make money. The extra fuel required to get you to your destination 30 minutes sooner is not cost effective.
05-25-2009, 09:01 PM
#24
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Joined APC: Jul 2008
Position: 747 Captain (Ret,)
Posts: 807
In my limited jumpseat experience, I've found that there is roughly a 10 to 15% reduction in power upon reaching cruise.
EX/On SWA yesterday, climb N1 was roughly 99% at top of climb. Reduced to 84% to maintain mach .75.
With many large aircraft, the FMS takes into account the wind, temp, cost index etc. and provides the crew with a rough power setting to maintain desired mach/IAS in both climb and cruise. In many aircraft, the auto throttles will maintain a selected mach number.
EX/On SWA yesterday, climb N1 was roughly 99% at top of climb. Reduced to 84% to maintain mach .75.
With many large aircraft, the FMS takes into account the wind, temp, cost index etc. and provides the crew with a rough power setting to maintain desired mach/IAS in both climb and cruise. In many aircraft, the auto throttles will maintain a selected mach number.
If a jet aircraft is forced by ATC to level off below 10,000', it could easily take a 70%-80% power reduction to remain below the 250 KIAS speed restriction.
Joe
05-26-2009, 07:47 AM
#25
Prime Minister/Moderator
Joined APC: Jan 2006
Position: Engines Turn Or People Swim
Posts: 40,382
In a turbofan engine, power is non-linear with respect to RPM. The CFM-56 on the B-737 has a flight idle of about 50%. A better approximation of actual power produced is fuel flow. At top of climb the fuel flow of a 737 with CFM-56 engines will normally be 4,000-5,000PPH. Normal cruise FF would be 2,500-3,000PPH. So there would be a 25%-50% power reduction when leveling off. This power reduction is most pronounced in two engine aircraft, and there is less of a reduction for three or four engine aircraft when operating near the aircraft's ceiling.
If a jet aircraft is forced by ATC to level off below 10,000', it could easily take a 70%-80% power reduction to remain below the 250 KIAS speed restriction.
Joe
If a jet aircraft is forced by ATC to level off below 10,000', it could easily take a 70%-80% power reduction to remain below the 250 KIAS speed restriction.
Joe
True. Turbofans produce 80% of their power in the top 20% of the N1 range. A small N1 reduction in the climb/cruise regime produces a large power reduction.
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