Unknown mass
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Join Date: Nov 2002
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Unknown mass
I am an electronic engineer, and look after equipment calibrations as part of my job. Yesterday I flew out of Fuerteventura, and noticed that the baggage weighing machine was not 'tare'd before my luggage was put on, reading -12.5Kg with nothing on there. I thought at the time that it was a bit stupid of the designer not to put an auto-tare sequence into the software, and thought no more about it. It was only today that I realised that there must have been a huge amount of extra weight that the aircraft was having to haul off the ground in that heat. I did some quick sums, and came up with about an extra 2.5 tonnes. The temperature was +28 degrees C. I was just wondering what kind of effect that would have on the 757-200 that we were on (I only fly PA-28s, so can't judge). I was also wondering what would happen to an aircraft with a little less performance, losing an engine just after V1, with an extra tonne or so on board?
Just curious.
Just curious.
Join Date: Nov 1999
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Although you place your bags on the scale very often the weight is not computed. It is common to use notional weights for passenger bags. If the flight was a charter flight it would probably have been in the order of 13 Kg. Of course your bag (like mine) is probably well above that, but averaged out over a full load that is the figure they come up with.
sometimes actual weights will be given or requested, and the weight divided by the number of bags ought to generate a sensible sort of cross check figure (usually 12 to 18 kgs).
A 757 FUE-LGW would typically have a ZFM of around 80,000kgs and a fuel load of 20,000kg giving a take off mass of (give or take a couple of tonnes) 100,000Kg. If there were an error of 2500 kg or 2.5% the difference to the take off speeds would only amount to about 2 knots (V1. VR and V2 ). given the inherent safety factors built in to the calculations the effect would be negligable., although I am not suggesting it would be unimportant !
many aircraft including the 757 use an assumed temperature method of reducing the take off thrust (by up to 25%) where runway length exceeds that required for a full power accelerate / stop distance. This is achieved by telling the thrust management computer that it is in fact warmer than ambient temperature. The maximum input for assumed temperature is around +54c . So as you can see 28c still leaves a lot of room for derating.
An engine failure after V1 would result in the application of maximum thrust on the other engine in any event and the aircraft would still be around 13000kg under its max certified take off mass in any event, and even then that would not take into account any safety margins built into the performance factors.
sometimes actual weights will be given or requested, and the weight divided by the number of bags ought to generate a sensible sort of cross check figure (usually 12 to 18 kgs).
A 757 FUE-LGW would typically have a ZFM of around 80,000kgs and a fuel load of 20,000kg giving a take off mass of (give or take a couple of tonnes) 100,000Kg. If there were an error of 2500 kg or 2.5% the difference to the take off speeds would only amount to about 2 knots (V1. VR and V2 ). given the inherent safety factors built in to the calculations the effect would be negligable., although I am not suggesting it would be unimportant !
many aircraft including the 757 use an assumed temperature method of reducing the take off thrust (by up to 25%) where runway length exceeds that required for a full power accelerate / stop distance. This is achieved by telling the thrust management computer that it is in fact warmer than ambient temperature. The maximum input for assumed temperature is around +54c . So as you can see 28c still leaves a lot of room for derating.
An engine failure after V1 would result in the application of maximum thrust on the other engine in any event and the aircraft would still be around 13000kg under its max certified take off mass in any event, and even then that would not take into account any safety margins built into the performance factors.
