View Full Version : Higher not always better?
vinayak
3rd Nov 2009, 09:52
Perhaps not the best place to be asking this, but couldn't figure where else to post it.
Yesterday flying from Bombay to Calcutta at FL330 had tailwind of about 40 knot. Over Nagpur, I asked the Captain if I may request for FL390 so we get more tail wind. The GS at this time was about 490 knot.
He asked if it would really be worth it... And asked me to request the controller for a climb. Which we did. Climb to FL390.
This time there was a tailwind of about 90 knot. I thought it would make me reach the destination a tad bit faster. To my surprise we lost 5 minutes and no change on the fuel. The GS at this time was about 515 knot
We were also, maintaining a constant M.78....
How did I lose 5 minutes despite being higher and in a stronger tailwind??
zlin77
3rd Nov 2009, 10:11
Maybe the T.A.S. decreased with increasing altitude?
vinayak
3rd Nov 2009, 10:19
Yes it did. To maintain the same Mach at an higher altitude the TAS required is less.
Regarding your fuel, you have to look at fuel used to climb against fuel saved at higher cruise level. Sometimes it is not worth it. Also, cruising above economic cruise level uses more fuel.
vinayak
3rd Nov 2009, 10:45
Econ was FL375... :confused:
There you go! Captain was right :). Why on earth choose 390?
You state that you lost 5 minutes and no difference in fuel.
How did you compare the outcome of the 2 situations? Are you talking about the FMC arrival estimates before and after the climb or are you comparing the actual arrival after flying at FL390 with the forecast figures that you had noted at FL330?
If you talk about both estimates, were the wind figures for the whole route FL390 inserted? If not, an estimate for a long flight to destination could be somewhat in error.
If you talk about the actual figures after arriving at destination, the difference could be in extensive approach vectoring, and the result from staying at FL330 could have been even worse.
And of course, as BOAC noted, if the remaining flight time was short, you may not have gained back the fuel used for the climb from FL330 to FL390.
Rainboe
3rd Nov 2009, 14:58
Nagpur was under 1 hour to TOD. Not enough time to make up for the fuel used in climb! If you picked up an extra 50 kt tailwind, you would have saved time. However, you must have dropped out of the high tailwind area. You should have most certainly gained time if not fuel.
Just a couple of points Vinayak:
How far to destination when you climbed? Less than 250nm and it's usually not worth it.
What did your OFP/CFP say about it? It usually knows best.
Did you insert all the winds, temps, trop etc either manually or via Acars in order to make predictions accurate?
What was econ Mach at 330 and 390 - M.78?
Please don't feel the need to reply, but the answer might be in one of the above points.
cheers
mcdhu
Vinayak - I still have 2 questions:-
1) Why on earth choose 390 which must have been close to 'Max' cruise? Why not 370?
2) Why did your Captain accept that? Was he trying to let you prove something to yourself?
muduckace
3rd Nov 2009, 16:06
Sometimes best to stick with the flight plan on short flights, worked MTC control in a small office of a small airline operating DC-9's. It was a constant struggle for the dispacher at my side, often on some of our really short legs it save fuel to cruise at fl280 for 15-20 minutes than climb to fl330 as the standard practices for that leg may have dictated.
galaxy flyer
3rd Nov 2009, 21:35
My 2 cents:
It hurt more to be 1500 feet above optimum level than to be 2500 feet below optimum. Being too high hurts badly--upon level off, did the power smoothly reduce to CRZ power or stay at CLB trying to maintain M.78?
GF
con-pilot
3rd Nov 2009, 21:52
The only number that needs to looked at in this situation is the GS. By maintaing a constant Mach and increasing alitutude from a lower flight level to a higher flight level will decrease TAS. So if the GS did not increase after the FL change the time you lost, if in fact it was a true loss of time, was probably caused by the climb to the higher FL and the loss of TAS.
Remember the fastest TAS for most types of jet aircaft is attained in the mid 20's Flight Levels. Of course the fuel burn is at the top of the scale at those altitudes, so you are giving up a lot of range/endurance. But if you're in a hurry and have fuel to burn, so to speak, that's the trick. At least in every type of jet I have flown, which is, eight entirely different types of jet aircraft, not counting the different series of each type. Such as the Saberliner, I didn't include all of the series, the 40, 60, 75, 80 and the 65, all of which I have flown.
fender
3rd Nov 2009, 23:28
Did you put the arrival runway in after you climbed?
framer
3rd Nov 2009, 23:47
It is hard to figure out without all the information.
If your groundspeed went from 490 to 515 as you say, it is indeed hard to imagine where you lost 5 mins. It certainly wouldn't have been in the climb to 390.
Personally I doubt that the flight actually took five minutes longer than it would have if you had stayed at FL330.
Basically i think the answer lies in where you are getting the information from that you lost 5 mins. I think it is not the right place.
Framer.
PS was FL350 and FL 370 not available?