Holding - Best FL for Jets
 

I have two questions...

Q1. What are optimum levels for holding Jets like B737?

Q2. Suppose one is approaching destination say at FL 400 and ATC tells expect delays
Now we have 2 possibilities
1. Go with minimum clean at higher FL and cruise longer and delay descent since Jet Engines are more effective at higher FL.
2. Start descent early with min clean and hold at lower altitude
Which is more cost effective?
Thanks

I think it is better to hold at a lower altitude. Fuel flow is a function of TAS, so for any given holding speed, your TAS will be lower the lower you are.....if you check the numbers on your aircraft, I am sure you will find the fuel flows lower at lower altitude(@ hold speed).

You mention the effieciency of a jet engine improving with altitude.....this is actually SFC that improves, which is a function of distance rather than time.

That said, I imagine that not many congested ATC units will give you a choice about where to hold, they will simply slot you into the queue and you go down in turn.

In terms of delaying or starting descent early, I(we) do neither. Just start the descent where the computer says to and that is that. Many times the hold is cancelled before you get there. It is however good practice to ask ATC if you may slow down, as you suggested.

"Optimum Holding Level" - theory is, for Endurance, it does not matter. In practice, for the A320 I fly, it is slightly better to hold at high altitude, but only a few % in it.

"Suppose one is approaching destination say at FL 400 and ATC tells expect delays... Which is more cost effective?" I presume you mean "what is least fuel consumption?." Min Fuel Flow is what you want, so:

1. Power to idle
2. Slow to Min Clean (Min Drag)
3. Descend (& Hold) clean at Min Clean

You will get a long time to await your turn to land at low fuel consumption.

Other factors clearly come into play:

1. Are ATC happy with the slow RoD? Holding at these FLs?
2. Are you sure you will land at Dest (or close to)? If a distant diversion is a possibility, then descending and needing to climb back is not a great idea...

It's partly energy management - PE (& KE). Use the PE wisely, flying at anything above Min Clean, or using the SB = throwing energy away which has to be replaced by fuel. Ditto, anytime thrust is above idle unnecessarily = wasted fuel.

It really saddens me to see any pilot worth his salt saying such things :{ It is not possible to "program" an FMC with all the variables required, and pilots should understand the basic aerodynamics and factors - no matter how clever an Airbus or FMC is, it cannot alter basic physics. I am saddened by colleagues who will not fly a visual approach, or have to program the damn FMC/FD to fly it - and are unwilling (unable?) to look out of the window and fly accurately and appropriately for the conditions and efficiency.

NoD

Best at 20,000ft ish in my Anglo-German-powered steed! ;)

"politics" can come into it. If you hold near the bottom of the stack then you make the approach earlier, so saving fuel and maybe getting in when the situation is variable.

Had this conversation last night. It depends upon the a/c weight.

Tab data in 5,000' increments below FL 290, 2000' increments above FL 290 -

777 @ 620,000 lbs? 10,000' 21,000' below OPT ALT
@ 600,000 lbs? 15,000' 17,000' below OPT ALT
@ 500,000 lbs? 25,000' 11,000' below OPT ALT
@ 460,000 lbs? 31,000' (MLW) 7,000' below OPT ALT
@ 400,000 lbs? 31,000' 10,000' below OPT ALT

Actually, fuel flow in a jet is more closely related to IAS. While not exactly constant with altitude, it is very close.

Depending on the airplane, higher altitudes (above 15 or 20,000') may require a higher holding IAS due to required stall margins (e.g., 15,000' for the 744). If so, it would be best to be below that altitude.

At max TOGW (~400 T), 744 holding is a constant 286 KIAS to 15,000', and fuel flow varies by 1% in that range. At 30,000', holding is 312 KIAS with approx 10% higher fuel flow.

At light weight (200 T), holding speed is constant 208 KIAS to 15,000', and increases to 222 KIAS at 45,000'. Lowest fuel flow is at 30-35,000'; and is 13% higher at 45,000', 8% lower at 1500'.

The last time I figured out most efficient altitude near landing weight was a B727. It was 25,000 ft. Above or below meant less holding time. I'm sure all types will be nearly the same. Staying high doesn't work for holding, only cruising because holding you don't care what your forward speed is.

On the 727 I just looked up the fuel flow at my weight holding at different altitudes and bet your airplane type will be close. It takes 2 minutes.

Agree with what most people have said on here.

In the B738W you really are splitting hairs to be too concerned at the level you hold at. There really isn't that much difference in your fuel flows between say FL300 - FL150. The best level will be the one (generally) that fits your profile when ATC clears you to exit the hold in my opinion. Obviously there will be other considerations such as traffic, ride, icing etc etc

But, that being said, if you were to hold for an extended period (more than say 30mins) and had a choice of level, FL250 would give you the lowest burn for a given weight. eg 2160 kg/hr @65T B738W