When determining performance on light single and multi-engined aircraft most of my mentors stated that it was unlikely you would achieve the performance stated in the manual as these figures related to a company test pilot carrying out the tests in a new aircraft in ideal conditions at known weights. Not too much of an issue, when flying privately in the UK at least, if the performance achieved does not match the performance published in the manual. This performance is factored for average aircraft and pilot performance.
Likewise jet transport aircraft are extensively tested by company test pilots carrying out the tests in a new aircraft, at known weights, with a manufacturer keen to prove to airlines that their aircraft performs as well as or better than its rivals.
When flying public transport on a daily basis pilots refer to standard performance tables, based on standard passenger weights in aircraft whose power output in taken as read. I have never known of any definitive testing of jet airliners at any other than theoretical weights. Who knows if jet engines always provide optimum power? Who knows if ALL the fleet will achieve comparable performance? Who knows if the performance figures calculated by the airline will be achieved in practice under all conditions in all aircraft? Has this EVER been proved? It would be interesting to take the actual weights of the passengers and compare them to the calculated standard weights to see if there is a significant variation.
The figures are taken as gospel, but unbeknown to most passengers the figures used to calculate performance are THEORETICAL - not ACTUAL passenger weights or PROVEN aircraft performance data. When was the last time YOUR aircraft was ramp tested to ensure it could achieve its book performance. Probably NEVER.
So a relatively old Spanair aircraft, flying out of a hot and high airfield, what performance was actually achieved?
It may be proved that the performance issue was not a determining factor in this accident, maybe only a contributory one. But next time you fly remember no one knows ABSOLUTELY what the weight of the aircraft is or can GUARANTEE that it will perform EXACTLY as stated in the aircraft manual. Safety factors built in to the calculations ensure aviation is a safe environment in which to operate and that aircraft don't fall out of the sky.
However, many aircraft have crashed in the past because of lack of this absolute knowledge of weight and performance and they will continue to do so in the future.
There are a lot of articles on the web calling for a change in the standard weight of passengers. Remember as an adult female in the UK and Europe the 'standard weight' is 70kg - That's about 11 stone. Take off hand luggage, which isn't weighed, the weight of your clothes, handbag and other items and the airlines probably assume the 'average' woman weighs around 9 stone. Similar calculations for men would result in a 'weight in your birthday suit' of 12 stone. So next time you fly and see people stuffing max size hand luggage in the overhead locker and see 'the fat family' sat next to you - or you weigh more than 12 stone as a man or 9 stone as a woman, the pilot will NOT have accounted for this excess weight.
Just do a quick quiz when your flying - look at the passengers, guess their weight and work out the excess. Makes you think?
Also check out
Google to see the articles calling for a revision to these THEORETICAL weights!
Any male passenger over 12 stone and any female passenger over 9 stone and any child under the age of 14 who weighs more than 5 stone, with hand luggage is compromising air safety.