Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

A320 - max endurance in the holding (constant altitude)

Tech Log The very best in practical technical discussion on the web

A320 - max endurance in the holding (constant altitude)

Old 5th Nov 2008, 12:38
  #1 (permalink)  
Thread Starter
 
Join Date: Apr 2005
Location: Planet Earth
Age: 44
Posts: 22
Likes: 0
Received 0 Likes on 0 Posts
Question 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)

Thank you for you answers
Wireflyer is offline  
Old 5th Nov 2008, 13:01
  #2 (permalink)  
 
Join Date: May 2000
Location: Glorious West Sussex
Age: 75
Posts: 1,020
Likes: 0
Received 0 Likes on 0 Posts
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.
TyroPicard is offline  
Old 5th Nov 2008, 13:15
  #3 (permalink)  
Thread Starter
 
Join Date: Apr 2005
Location: Planet Earth
Age: 44
Posts: 22
Likes: 0
Received 0 Likes on 0 Posts
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.
Wireflyer is offline  
Old 5th Nov 2008, 13:44
  #4 (permalink)  
 
Join Date: Jul 2007
Location: UK
Posts: 41
Likes: 0
Received 0 Likes on 0 Posts
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!!!
Cheque List is offline  
Old 5th Nov 2008, 13:47
  #5 (permalink)  
 
Join Date: Jul 2000
Location: My views - Not my employer!
Posts: 1,018
Likes: 0
Received 0 Likes on 0 Posts
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 ) but I would think green dot to be your friend.
Cough is offline  
Old 5th Nov 2008, 13:57
  #6 (permalink)  
 
Join Date: Jul 2008
Location: south pacific vagrant
Posts: 1,333
Likes: 0
Received 0 Likes on 0 Posts
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!
waren9 is offline  
Old 5th Nov 2008, 16:12
  #7 (permalink)  
 
Join Date: Jan 2008
Location: Finland
Age: 76
Posts: 465
Likes: 0
Received 0 Likes on 0 Posts
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!!
finncapt is offline  
Old 5th Nov 2008, 17:48
  #8 (permalink)  
 
Join Date: Sep 1998
Location: wherever
Age: 53
Posts: 1,616
Likes: 0
Received 0 Likes on 0 Posts
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.
FE Hoppy is offline  
Old 5th Nov 2008, 18:00
  #9 (permalink)  
 
Join Date: Dec 2006
Location: madrid
Posts: 86
Likes: 0
Received 0 Likes on 0 Posts
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
avionneta is offline  
Old 6th Nov 2008, 05:36
  #10 (permalink)  
 
Join Date: Nov 2007
Location: Asia
Posts: 7
Likes: 0
Received 0 Likes on 0 Posts
And where is this graph available to view or get hold of??
Derate is offline  
Old 6th Nov 2008, 13:15
  #11 (permalink)  
 
Join Date: Sep 1998
Location: wherever
Age: 53
Posts: 1,616
Likes: 0
Received 0 Likes on 0 Posts
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.
FE Hoppy is offline  
Old 7th Nov 2008, 20:28
  #12 (permalink)  
 
Join Date: Dec 2006
Location: madrid
Posts: 86
Likes: 0
Received 0 Likes on 0 Posts
i've got that graph and can send it on request. it's in french
avionneta is offline  
Old 7th Nov 2008, 20:47
  #13 (permalink)  
 
Join Date: Jul 2006
Location: USA
Posts: 451
Likes: 0
Received 0 Likes on 0 Posts
Avionneta is basically correct....

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

Fly safe,

PantLoad
PantLoad is offline  
Old 7th May 2022, 22:30
  #14 (permalink)  
 
Join Date: Dec 2006
Location: Athens
Posts: 32
Likes: 0
Received 0 Likes on 0 Posts
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




These results show that jet aircraft have to fly about 9% faster than minimum drag speed to achieve minimum fuel consumption.

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

Last edited by pilot-737; 7th May 2022 at 23:43.
pilot-737 is offline  
Old 8th May 2022, 13:11
  #15 (permalink)  
 
Join Date: Aug 2013
Location: The bush
Posts: 248
Likes: 0
Received 0 Likes on 0 Posts
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
The Banjo is offline  
Old 9th May 2022, 07:53
  #16 (permalink)  
fdr
 
Join Date: Jun 2001
Location: 3rd Rock, #29B
Posts: 1,864
Received 0 Likes on 0 Posts
Originally Posted by pilot-737 View Post
If you are talking...

SFC vs Thrust



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

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information

Copyright © 2022 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.