B744 take off performance
 

Hi, hope you can help. This has puzzled me for a while now.

A couple of years ago, I flew from Heathrow to Brisbane via Kuala Lumpur. I can't remember why, but I timed the take off rolls.

LHR to KLIA 14 hr flight - 55 seconds.

KLIA to BNE 8 hr flight - 65 seconds.

I was just wondering why the take off roll at LHR was less, despite carrying more fuel for the longer leg. The a/c was full of pax on both legs. The r/w at KLIA is about 3000' longer than at LHR. Is it just that less engine power is used/reqired to reach Vr due to that extra 3000' ?

Cant say for sure, but I think surface winds at both aerodromes were calm.

Thanks in advance,

AV8

my guess is OAT which in turn affects air density and thrust.
another reason could also be that reduced thrust was used on the second t/o to take advantage of the longer r/w.or
II segment climb perf at LHR or obst on t/o path or
flap configs or
....got to go...;)

Have to get off the deck asap at LHR!!!!!!

Any idea how much freight on board?

Tankering fuel?

Soooo many possibilities...

Felix,

This seems to contradict the concept of reduced thrust, so would you be so kind as to supply an official verification for your statement.


Thanks.

Mutt

Thanks for the replies guys.

I didn't realise that there could be any number of possibilites. I'll just have to accept that I'll never know!

Cheers

AV8

Simple...the use of reduced thrust
 

Most operators are approved to use reduce thust on departures, some as high as a 20% reduction from maximum thrust, with governmental, manufacture and operator approvals. This is all very safe and a good way to operate. Taking an engine to 100% expose the operator to a statically high expose to engine failure than a reduced thrust. Do you accelerate you car to max rpm before every shift? Of course not, I hope.

So timing the departure roll could be very missleading.

Hope this helps.

What about an improved climb? Don't know anything about KLIA or B747 performance specifically but if obstacles are an issue the the longer runway allows for a higher decision speed V1 and therefore a higher climb speed V2, with an associated improvement in climb performance.

Da, the orignal question only had to do with time spent on a runway, not with improved climb or clearing a rock 30 km from the airport. Most climb performance has to do with the ecomonies of controling the energy and nothing to do with the safe operation of the flight.

Heathrow is very noise conscious so chances are that it would have been a max thrust departure in order to get the shortest take-off roll and a good initial climb. Because you achieve height quicker it tends to be a quiter departure even with max thrust.

I'm not sure about KLIA but I suspect that noise is not such an issue and together with the longer runway, I suspect that they would have reduced take-off thrust. On the 744, take-off thrust can be reduced in excess of 20% (54 degrees assumed temperature with TO2 rating). I doubt however that they would have used a full derate, probably something in between.

Regards

csd

csd,

Would you be so kind as to point out a technical reference showing that this is true or required?

As for reduced thrust, talking about the B747-400, it generally comes with 3 thrust ratings, TO/TO-1/TO-2, the thrust reduction is selected by the operator but 0%/10%/20% are common. There is also another procedure called Assumed Temperature Thrust reduction, this permits a reduction of 25% of the RATING in use, so combining TO-2 with Assumed Temperature, you can actually reduce the engine thrust by approx 45%!

AV8, you will never know what happened on that day :)

Mutt

Hi Mutt

Sadly can't produce a technical reference but within my company it is a requirement that above a certain weight (at certain noise sensitive airports) we go max thrust. My understanding is that the shorter take-off roll together with the relatively high initial climb keeps the noise nearer the airport and with the gain in height the noise is then diluted more. Climb thrust is then selected early (1000' agl) so that by the time you are crossing the noise monitoring stations you have the added height together with the reduced thrust for climb rather than being lower and still with take-off thrust (albeit reduced).

I'm led to believe that this reduces our noise violations. Another airport where we use a similar policy is Brussels. Bear in mind that we only do this for heavy weight departures and where noise is an issue.

Regards

csd