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Macsm

Flight idle and descent planning...

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Hi,If this sounds illogical, it probably is, but here it goes: By far, my favourite part of the flight is descent planning; so was just curious as to how this is done in real life. I always use the rule of 3 for TOD distances (FL x 3/100)+/- 10% per 10 knots of head/tail wind, and I can get pretty accurate. I use the mach .72/310 knots profile, but am not sure what role does flight idle or no flight idle power settings play on real life operations. I can see several advantages for not using flight idle:1.) Less spool up time when needed2.) Minimum hydraulic and pneumatic pressure vaileable?3.) In newer engined 737's (CFM's) it is prhibited to enter high rain areas with Flight idle as I undesrtand..Disadvantages;1.) Economy?2.) Economy?3.) Economy?Any thoughts on this? Just curious,regards,Macs :)

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>1.) Less spool up time when neededNot really...it's not likely you'll need to rapidly push it up from a descent. I only consider that a factor when dirty and close to the ground.>2.) Minimum hydraulic and pneumatic pressure vaileable?Hydraulics...not really. But airflow...especially if you have any anti-ice turned on, is a factor here. Depending on how "tight" the jet is, you could find the cabin actually climbing due to insufficient incoming air if you pull 'em all the way back.>3.) In newer engined 737's (CFM's) it is prhibited to enter>high rain areas with Flight idle as I undesrtand..That's due to the very effective centrifugal slinging of liquids and solids out to the bypass duct on a hi-bypass fan. But the JT8D is a fairly low bypass engine...have never seen that limitation on the -200.>>Disadvantages;>1.) Economy?>2.) Economy?>3.) Economy?All of the above ;-)CheersBob ScottATP IMEL Gulfstream II-III-IV-V L-300Santiago de Chile

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>Hi,>If this sounds illogical, it probably is, but here it goes: By>far, my favourite part of the flight is descent planning; so>was just curious as to how this is done in real life. I always>use the rule of 3 for TOD distances (FL x 3/100)+/- 10% per 10>knots of head/tail wind, and I can get pretty accurate. I use>the mach .72/310 knots profile, but am not sure what role does>flight idle or no flight idle power settings play on real life>operations.the normal descend speed from the 737-200 is .72/280/250.with Landing weight between 70 - 90 thousands pounds @ FL350, you will have an average Glide path of 3above 90,000lbs, 3.2 (3.5 @ MLW of 105,000lbs) of the glide path during normal descendSO, what is normal decent in turbo-fan jets?is descent in Idle Thrust>I can see several advantages for not using flight>idle:>1.) Less spool up time when needed>2.) Minimum hydraulic and pneumatic pressure vaileable?>3.) In newer engined 737's (CFM's) it is prhibited to enter>high rain areas with Flight idle as I undesrtand..1.) The average spool time from Idle to TOGA/MCT thrust is near 8 to 10 seconds.2.) Far as I know, a 737 use electric acumulators to give the hydraulic pressure. There is no direct link with the Thrust-Hydraulic. Just engine runing or not.3.) In all 737 with A/T, when you use WING and ENG ANTI-ICE, the a/t will provide the minimun N1% to keep the pneumatic pressure to suply pressurization AND Anti-icein -200, without A/T, is mandatory BEFORE apply the anti-ice, accel you engine to 55% of N1.The aircraft will ALWAYS make deviations to be out of the storm or heavy rain areas.The second one is more dangerous because can make "black holes" in the WX radar. You can identify this area when the surrounding area is big green area.RegardsGustavo

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>2.) Far as I know, a 737 use electric acumulators to give the>hydraulic pressure. There is no direct link with the>Thrust-Hydraulic. Just engine runing or not.>Regards>>GustavoThe 737-200 has 2 engine driven and 2 electrically driven hydraulic pumps. Engine driven are 'A' system and electrically driven are 'B' system. With both engines at idle there is more than enough pressure and flow from the EDP (Engine Driven Pumps) to supply the needs of the 'A' system.This configuration is different on the -300 and up.Cheers,JohnBoeing 727/737 & Lockheed C-130/L-100 Mechanichttp://www.sstsim.com/images/team/JR.jpg

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