View Full Version : Lo-o-ong T/O roll

12th Jun 2008, 00:22
Take-off time from KUL in a Malaysia 747: the roll seemed to go on forever before rotating. I assume that 30 degree temps and high humidity were the major factors, along with the fact that the a/c was full and had a 12-hour flight ahead of it and so was full of fuel; I believe KUL is close to sea-level. Just how much longer was it, compared to the standard 20 degrees at/around MSL?

12th Jun 2008, 01:23
:sad:If a takeoff roll lasts longer than 60 seconds its time to brace and KYAG :eek: (kiss your A$%E goodbye):oh:

Final 3 Greens
12th Jun 2008, 07:10

In these times of high fuel costs, you may also have experienced a reduced thrust take off, which also saves engine wear.

All calculated before departure and all safe.

As to how longer, I couldn't say, I'm not an ariline pilot.

Yellow Sun
12th Jun 2008, 09:41
In these times of high fuel costs, you may also have experienced a reduced thrust take off, which also saves engine wear.

The following refers to jet aircraft and does not necessarily apply to piston engined or turboprop aircraft.

Reduced/Flex/Factored Thrust take offs are the norm and have been for many years (I first started doing them 30 years ago). The chances are that almost every take off you have experienced as a passenger in the last 20 years has been subject to some degree of thrust reduction.

As to your question:

Just how much longer was it, compared to the standard 20 degrees at/around MSL?

I am afraid that that is very difficult to answer. Time is not a parameter that is calculated or derived in the takeoff calculation. Neither is the distance from brakes off to rotate (Take Off Run Required - TORR); from which time could be calculated with knowledge of the rotate speed; calculated separately, as this is normally taken into account by the "Balanced Field" method of performance calculation where the Take Off Run and Take Off Distance Available is reduced to the lower of the two figures. I shall not go any further into take off performance calculation, as whilst it is not rocket science or even quantum mechanics, it can rapidly come to resemble pages of flute music!

If you had the page from the 747 performance manual for the appropriate runway at KUL, then by inspection and guesstimate you could probably come up with a figure of sorts. How accurate it would be I really could not say.

I hope that this helps to some extent.


12th Jun 2008, 14:10
YS - thanks for those thoughts.

ISTR from my now long-gone PPL days that the key issues when determining length of T/O roll were air density and weight, with the former determined by altitude, humidity and temperature. I'm not sure to what extent they affect a modern commercial jet and of course I wasn't timing the roll with which I started this thread, so my report can only be subjective. It didn't seem to use as much acceleration as I'm accustomed to though so I assumed (it now turns out possibly wrongly) that this was in the plan (reduced thrust) rather than a result of low air density, resulting in lower engine thrust.

But it's interesting that take-offs are routinely performed with less than max thrust: I was taught that runway in front of you is way more valuable than runway behind you, so the quicker you take-off, the better. Maybe there's a difference between a 747 and a Cessna 152...

12th Jun 2008, 16:11
Non-pilot speaking
With high altitudes, as well as temps, when departing from JNB, I have known 60 and 65 seconds before rotation in a 744. Whereas, from LHR, 45/50 seconds might be considered 'typical'. From all of these fields, the range of variables is very well known.

13th Jun 2008, 10:44
Have to agree with PAXboy.... flew out of Bogota and Quito many times (the former around 8500ft, and not sure about Quito, but I think it's even higher), both Runways very long, lengthy take-offs... used to think they had enough time to show an in-flight movie and a meal on the take-off run... :)

FL370 Officeboy
13th Jun 2008, 11:09
Well the differences between a Cessna 152 and a B747 are many - however one particuarly relevant here is that the B747 (being a large public transport airliner) is certified under performance category A.

That is to say, that if an engine fails on the take off roll, you must stop if you are before 'V1', and continue into the air if you are beyond 'V1'. The aircraft is certified as such that either course of action should lead to the aircraft still being controllable by the pilots.

In a Cessna, if your engine fails on take off you are going down. In a Class A aircraft, your remaining engines will meet thew climb requirements. In the case of a reduced thrust take-off, you may reach V1 at a different place on the runway, however the aircraft will still meet all the safety criteria of it's certification. A reduced thrust take off is standard in all airlines that I know of, with certain parameters where full power takeoffs are mandatory (first flight of day, every 10th flight, Anti-ice on etc etc). I will be taking off later on today with about 80% power.

*V1 = take off decision speed. Above V1 = GO, below V1 = STOP. At certification, this is the speed at which the critical engine is assumed to fail.

13th Jun 2008, 11:12
There was a time, many years ago, I used to time rolls (I lead a sad life:) ). I recall times ranging from 54 to 56 seconds on B747 (200 series) for MIA-LHR and SIN-LHR.

13th Jun 2008, 13:50
FL370 - interesting info there for a non-commercial pilot, thanks. Would the first take-off of the day be on full whack in order to ensure the engines are up to it? What's the rationale?