B777 Holding Altitudes

[BMM389EC]

Hello All.
I was having a look at the Holding table in the FCOM and came up with the following:
It seems the heavier the aircraft is, the lower fuel flow per engine is achieved at a LOWER altitude. i.e 240T-15000ft-3350, 220T-20000-3050 etc. Now I thought if you were going to hold you'd be better off holding as high as you can provided you're not way above optimum. Only at 160T do we get up to 30000.
These numbers are for a -200 and the same principle applies on the -300 table-just the numbers are different obviously.
Now, either I'm missing something or I've forgotten some theory I should know, but I'm not sure if I understand this!?
Any ideas?

[flightleader]

Think again,the holding speed would differ at different altitude thus different fuel flow.Please refer your Jepps for ICAO or FAA max holding speeds table.

[singleseater]

Best holding is to do with min fuel flow. This is normally achieved at V MIn Drag or best L/D ratio.
So the best holding speed in the 777 is normally just above the amber line, which is independent of alt. (IAS/CAS). (Same principle applies to drift down speed)
However, Drag is relative to TAS so the higher you go, the higher the TAS for the same IAS/CAS speed, and so the higher the drag. Obviously, at lower alts, the F/F is higher so there is a cross over and that is best hold.

[SuperRanger]

hi BMM,

i guess in this case there are 2 variables i.e. weight and altitude. actually, the 3rd variable would be the holding speed but i guess in this case it assumes the best holding speed from FMC.

for a given weight, you will noticed the best FF will be achieved at a certain altitude i.e. optimum altitude. if you move away from that alt, either up or down, the FF increases.

so, in your example for 240T the opt holding alt is 15,000' whilst for 220T the opt holding alt is 20,000'. obviously, the FF for 240T will be higher than 220T i.e. 3350 Vs 3050.

hope this helps...

SR

[Thylakoid]

Done performance on 777 long time ago, but there is something about FL250 being the best holding altitude for the B777. Corrections/inputs anyone?
Boeing tables are usually very precise; they generate them from the "B" tests.

[triathlon]

i agree. boeing knows the numbers

[gimmesumvalium]

Wow, BMM, am impressed with your astuteness. What were you flying before, a Cherokee 140???

[Bosche]

Originally Posted by singleseater

Best holding is to do with min fuel flow. This is normally achieved at V MIn Drag or best L/D ratio.
So the best holding speed in the 777 is normally just above the amber line, which is independent of alt. (IAS/CAS). (Same principle applies to drift down speed)
However, Drag is relative to TAS so the higher you go, the higher the TAS for the same IAS/CAS speed, and so the higher the drag. Obviously, at lower alts, the F/F is higher so there is a cross over and that is best hold.

B777 Amber speed line isn’t effected by alt?? Hmm

[324906]

For what it’s worth, if I remember correctly, the B737-8 anywhere between FL410 and 3000 ft, the difference in hourly burn was about a cup full an hour, from the performance inflight bit of the QRH

[Capn Bloggs]

That's a record: 12 years between posts!

[Sidestick_n_Rudder]

Originally Posted by Capn Bloggs

That's a record: 12 years between posts!

That's called 'necroposting'

[324906]

I think I’m a slow thinker, will let you know at the end of the next decade, after I’ve thought it through