A320 - max endurance in the holding (constant altitude)
 

Hi everybody,
here's a question I`m thinking about:

Situation:
You are cleared down to a e.g. 5.000 ft lower altitude with a clearance to proceed to a holding pattern which is lets say 50 NM away. You receive a EAT which is in e.g. 45 min. - so you will definitely end up doing your turns in the hold.

What are the most efficient speeds for this situation?

My suggestion:
- Green dot with idle power during the descent, since L/D max "without" (or with idle) thrust gives you minimum drag or minimum fuel flow.
- Upon reaching the cleared altitude which you have to maintain to enter the hold, increase speed to CI=0 to receive maximum endurance.

During climb CI=0 gives you maximum thrust available versus thrust required, ergo maximum rate of climb if needed. In cruise it gives you max endurance.
So why does everybody tell you to hold (in constant altitude) with green dot speed? (lets assume max. holding speed rule does not apply) :confused:

Thank you for you answers

If you know you have to hold for xx minutes, and you will reach the holding fix within that time, the only thing that matters is minimising fuel flow/fuel used.

Yes T.P., that's exactly what I want to know:
Does green dot or CI=0 minimize your FF in the hold in constant altitude?
W.

Green dot will always give you minimum fuel flow in the descent phase (doesn't matter if you're level or not).

Cost Index speeds assume that you will make a model flight & approach. Descending at the perfect time, and making a constant descent to the runway. In this case you are not. As soon as a you have been given an EAT which delays you, the best course of action is to go back to green dot to minimise fuel flow. (You might consider holding at higher levels to again reduce the fuel flow if possible, eg. quiet airports).

Cost Index = 0 would be more appropriate if you knew as soon as you took off that you were going to have a delay because of a night time jet ban at the arrival airport etc, although it wasn't designed to be used as such, becuase it is a management tool to optimise costs rather than a speed controller. - Hope this helps. CL

Easiest answer: Use managed!!!

In my book CI-0 is min fuel flow per nm. When holding you want min fuel flow per hour. I'm not yet Airbussed (soon to be :ugh:) but I would think green dot to be your friend.

CI 0 does not give max endurance. It gives minimum trip fuel. Thats why it commands best time to climb as jets burn lots down low.

Green dot is best angle. Best angle (Vx) is the IAS where there is the most excess of thrust avail over thrust reqd.

This is not the same as, but is close to the bottom of the drag curve, i.e min power reqd. (TAS)

Therefore green dot is close to max endurance.

Before anyone shoots me down, I've had a couple of gins!:ok:

I have been retired for a while now but, if I remember correctly, whilst green dot may theoretically be the best holding speed it does cause the a/throttle to move up and down chasing speed especially in turns.

I used to hold a few knots (5-10) above greeen dot which seemed to reduce this tendency and in my opinion probably saved fuel in the long run.

Theory and practice!!

in theory you want to hold a few knots above Vmd with AT off. Let the speed bleed off in the turn so when your wings level again you are at Vmd with a bit off excess thrust which during the straight leg will accelerate you back up to your few knots above Vmd for the next turn.

Thrust remains constant and therefore no over fueling to accelerate the engines.

Of course Vmd isn't published so Vx is probably your best bet and I think on the bus that would be green dot. Or if it isn't then Vfs would be a good speed.

and the answer is :
what you need for holding is best lift drag ratio. that speed is NOT green dot with two engine (with one yes), it is green dot + roughly 20 knots.
it's visible on a graph airbus doesn't publish as standard doc for crew. not a graph that falls in the need to know area but only in the nice to know

And where is this graph available to view or get hold of??

Avionneta,

If you have the graph could you compare it to a) Vfs (a good time to have C/L max) and b) Driftdown speed (another good time to have C/L max)

Thanks.

i've got that graph and can send it on request. it's in french

Avionneta is basically correct....
 

The flight Crew Training Manual addresses this issue. Please read for yourself.

Fly safe,

PantLoad

If you are talking about the “most efficient speed” in this scenario and you mean minimum fuel consumption (not minimum time in the holding), you have to think out of the box.

As long as your engines are operative, you have to think about the most efficient speed regarding the sum of aircraft (lift / drag ratio) and engine (most efficient setting, which is not idle for sure).

During descent, every level off, penalizes your fuel consumption, so it’s more efficient to leave your holding in order to commence you approach, without any level segment. That means you should delay your descent, taking into account the additional track miles of your holding patterns, or you should use some thrust(taking into account the additional track miles) as soon as you have been informed about the need to proceed in the holding pattern(use of V/S mode instead of idle thrust).

Of course in the real world, you will not always be the only one in the holding pattern, so an uninterrupted holding descent is not always possible.

Now regarding the holding speeds, as mentioned in the FCOM:

The default hold speed is the lowest of the:
‐ Maximum endurance speed
‐ ICAO limit holding speed
‐ Speed constraint (if any).
When no specific speed limit applies, the default hold speed is approximately equal to:
‐ Green Dot speed on the A318, A319, A320 (CFM) and A321
‐ Green Dot +20 kt on the A320 (IAE)

The sum of aircraft + engine may result in a speed higher than the green dot speed(you lose some fuel from increasing the drag above the max Lift/drag speed, but the penalty is offset by increasing the N1 to a more efficient fuel consumption range).

SFC vs Thrust

https://cimg3.ibsrv.net/gimg/pprune....bec15efe46.png



With a green dot speed of about 200Kts, the +20 Kts mentioned in the FCOM is around 9%.


(You can find the complete study here : https://www.fzt.haw-hamburg.de/pers/...TextBensel.pdf ).

This is the proof that descending with some thrust(and green dot speed), may be more efficient if a sudden information regarding additional track miles is received.

In addition, if your holding is close to the airport, and you are sure about the accuracy of your EAT, there is no reason to hold at the highest possible level in order to reduce your fuel consumption and be forced to use speed brakes in order to get a shortened approach.

To Summarize try to think in a similar way as the FMGS calculates the descent profile.

Step one : try to predict the track miles from the runway to the holding pattern. This will give you your the ideal altitude to leave the pattern.
Step 2 : According to your EAT, and your estimated entry holding pattern, calculate how many turns you will have to execute.
Step 3 : Add the additional track miles of your holding patterns to your remaining distance in order to adjust your descent.

The most efficient way would be to ask from the ATC to interrupt your descent close to your present altitude, and perform an uninterrupted descent in the holding.

if avionneta FE Hoppy or any other member can send me the graph, I would appreciate it.

Thanks

The calculation of green dot is a bit rubbery at best.
A320 2 x Wt plus 80 or 1.5 x wt plus 110 for the A321 (plus altitude adjusts.) Hardly a figures to hang your hat on. Then you have to consider an acceleration in most cases to exit the hold at 250 kts; and how to achieve that efficiently. And what of performance factors for individual airframes. Does this influence the speeds?

All very theoretical and not necessarily practical in the real world

Wow, -70/70A is not an engine that normally is associated with the term "efficiency". it gets worse when you add these blenders to an airframe... and then the very best use for them is to use them as a backdrop for some car show. The figures are so bad, that when we looked at that airframe to operate it, the performance dafa got a double take. PW made some good engines, and even made some that worked well on the 4 holers, but that wasn't one of them. The only good news was that there were oodles of spare engines available, they are essentially a rebadged -59, which was a better equipment mix.