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I watched a video of an A318 taking off at London City and it took just 20 seconds from setting takeoff thrust to getting airborne.

BA A318 takeoff @ LCY (takeoff thrust @ 1:23, airborne @ 1:43)

and here's A319 takeoff in a regular 3.5km runway, taking ~38 seconds

S7 A319 takeoff @ OVB (takeoff thrust @ ~3:26, airborne @ 4:04)

What I'm asking is a thought experiment, let's say, we fly from one airport to another, and everything is the same except runway length. In one case it's 1500 m, in another it's 3500 m.

How much thrust does an A318 (or any other aircraft certified to fly at LCY) apply at a regular airport vs on a short runway like LCY? How much thrust is used (in kN) and what is the thrust/weight ratio?

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    $\begingroup$ Related: you may refer to here where I did a calculation about the thrust (and energy) to get a 747 airborne. $\endgroup$ – kevin Mar 5 '17 at 13:59
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    $\begingroup$ This isn't a simple answer. It depends on the aircraft loading, engine configuration, air temperature, runway length, safety margin, etc. There isn't a set number for an A319 or A320 for example. The FMS calculates takeoff thrust when the pilots input various parameters. $\endgroup$ – Ron Beyer Mar 6 '17 at 3:31
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    $\begingroup$ @RonBeyer I understand. I'm not asking about something very precise, but rather a comparison, what if everything else is the same, except runway length. $\endgroup$ – culebrón Mar 6 '17 at 12:21
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Let's say there is an infinite runway, and a twin-jet capable of flying the whole route on one engine.

The jet starts its takeoff roll at maximum thrust, and at 40 knots an engine quits. Does the crew continue or not?


If they continue, they'll veer off the runway. Because the asymmetric thrust is huge and the airspeed is low, they won't be able to keep the plane on the centerline with the rudder.

Enter Vmcg

Vmcg is defined as the minimum speed, whilst on the ground, that directional control can be maintained, using only aerodynamic controls, with one engine inoperative (critical engine on two engine airplanes) and takeoff power applied on the other engine(s).

Why is it important?

The lower the thrust on short runways, the more you can carry...

It may be counter-intuitive, but here's why: there are three types of thrust reduction on takeoff:

  • Derate
  • Assumed temperature (aka flex)
  • Derate + flex

(Availability of the modes depends on the options available for the type and the options chosen by the airline for their fleet.)

Derate vs. flex and effect of derate on takeoff weight

enter image description here

The minimum control speeds associated with the FLEX take-off are related to VMCG/VMCA at TOGA thrust. Therefore, should the aircraft suffer an engine failure at V1, there is no limitation on selecting TOGA thrust on the remaining engine(s).

DERATED THRUST TAKE-OFF

A reduction in take-off thrust leads to lower VMCG/VMCA and hence, a lower V1. When taking off from short or contaminated runways where ASDA [Accelerate-Stop Distance Available] is the limiting factor, a reduction in the minimum control speeds may generate a take-off performance benefit and a higher MTOW.

As opposed to a FLEX take-off, the selection of TOGA following an engine failure during a derated thrust take-off is prohibited as long as speed remains below the first flap/slat retraction speed for the take-off configuration. The selection of TOGA below this speed in these configurations may result in loss of control due to VMCA considerations.

A330/A340 Flight Crew Training Manual

Taking off from LCY

A BA A318 taking off from LCY headed to New York with lots of fuel might as well be using reduced (derate) thrust because of this shorter runway.

In short, it's very hard to compare even if the only variable is the runway length.

Flex example

The CFM56-5B3 jet engine (powers the A321) can reduce thrust by 35% when a Δ35°C assumed temperature is used. So 120 kN of thrust becomes 78 kN (per engine).


Further reading:

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Takeoff thrust is calculated by a computer (or by looking up tables), since a lot of factors are considered:

  • Takeoff weight
  • Effective runway length
  • Runway surface condition (dry / wet)
  • Air temperature

"Effective runway length" is calculated from runway length, wind direction, wind speed, and runway slope.

Since we do not know the takeoff weight of the specific flights, I will make an assumption here and set it to 80,000 KG, which is the highest figure I have in the reference table.


At a temperature of 20 degrees Celsius, takeoff weight of 80 tons, effective runway length of 3000 meters, the speeds from the table are: V1 at 145, Vr at 153 and V2 at 155. This is the takeoff with max thrust, which according to Wikipedia, each engine is delivering 120 kN of thrust. The thrust to weight ratio would be 0.15. Converting the variables to SI units, I calculated that the time to reach V1 to be exactly 10 seconds.

This calculation is, obviously, far from reality, as it assumed:

  1. Thrust application is instantaneous.
  2. The aircraft maintained a constant 1.5 ms-2 acceleration. In reality, aerodynamic drag will come into play as the aircraft picks up speed.
  3. Max thrust is used on takeoff, which is rare in operations.

Moving on to shorter runways, the shortest runway length in the table I have is 2250 meters. It states that at this length, the maximum takeoff weight is 74.2 tons, and the limitations which resulted in this figure is runway length and Vmu(Minimum Unstick speed), the lowest speed at and above which the aircraft can safely lift off the ground with sufficient control surface authority and without a tail strike.

From the few data points I have, it seems that on a 1500 meters runway, the maximum takeoff weight would be around 63~65 tons.

enter image description here

So to answer your question:


  • If they used the same thrust, then the takeoff weight on the short runway is around 78% of that on the long runway.
  • If they kept the same weight, that would mean the pilots have extra thrust at their disposal on a longer runway. They can choose a derated takeoff thrust from the options -4%, -8%, -12%, -16%, -20% and -24%.

Source: A318/A319/A320/A321 Performance Training Manual

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    $\begingroup$ The question is asking about takeoff power levels, and your answer seems to be mostly about weights and speeds. I know they're related, but you don't seem to really address different thrust levels until the very end here, and only briefly. $\endgroup$ – fooot Mar 6 '17 at 17:17

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