Genghis the Engineer
5th Oct 2004, 13:58
I'm working up some steam to write a paper and cause the CAA a few headaches, so in the spirit of democracy I'd appreciate any opinions from my esteemed colleagues.
In bigger (Perf A or B) aeroplanes, it's normal to present WAT (MAT) curves for the aircraft's field performance. Based upon test conditions and actual conditions - wind, weight, surface conditions, slope. This is all good stuff, is well understood (at least by performance Engineers and professional pilots), and works well enough that field performance screw ups hardly ever happen.
However, for light aircraft (I'm using a deliberately vague term, since the precise terminology changes between authorities) it is usual to quote performance at MTOW/ISA/still air. Fair enough - but they if you look at the standard performance advice from authorities such as CAA (I think it's the same for FAA, but perhaps somebody more familiar can confirm or deny) the only factors given are adverse factors. Tailwind, adverse slope, wet surface, weight above MTOW (although why they give this latter always defeats me) - you get the picture.
In reality we take off into wind, or downslope very often. We all understand that temperatures below ISA improve field performance and so on - but the CAA at-least declines to offer any meaningful advice on the use of proverse factors.
So, routinely what happens? Where pilots bother to do field performance calculations they may often come up with a field length that is rather longer than the runway available. However, applying some common sense they realise that they've got a reasonable headwind (not included in CAA's factors), a bit of a downslope (ditto), and it's a sub-ISA day (again, not in CAA's factors). So, they know from personal experience that they've got enough runway, and proceed to fly.
Except, that they aren't doing this on any rational basis - they are relying upon "gut feeling". I don't like this, because gut feeling isn't all that reliable where aircraft performance is concerned and sooner or later somebody'll go through a hedge using gut feeling, rather than a meaningful field length calculation (the odds are that they already have).
There's a simple solution to this (in my opinion), which is that documents such as this one (http://www.caa.co.uk/publications/publicationdetails.asp?id=1161) should cover all aspects of factoring light aeroplane performance - including allowing for all factors that improve performance.
I don't see that this should create any unacceptable reduction in safety since there are "mandatory" safety factors applied at the end anyway - but it will at-least mean that performance estimates are reasonably meaningful, and pilots will see some benefit in proper field performance calculations, as well as being equipped to do so.
Thoughts anybody?
G
In bigger (Perf A or B) aeroplanes, it's normal to present WAT (MAT) curves for the aircraft's field performance. Based upon test conditions and actual conditions - wind, weight, surface conditions, slope. This is all good stuff, is well understood (at least by performance Engineers and professional pilots), and works well enough that field performance screw ups hardly ever happen.
However, for light aircraft (I'm using a deliberately vague term, since the precise terminology changes between authorities) it is usual to quote performance at MTOW/ISA/still air. Fair enough - but they if you look at the standard performance advice from authorities such as CAA (I think it's the same for FAA, but perhaps somebody more familiar can confirm or deny) the only factors given are adverse factors. Tailwind, adverse slope, wet surface, weight above MTOW (although why they give this latter always defeats me) - you get the picture.
In reality we take off into wind, or downslope very often. We all understand that temperatures below ISA improve field performance and so on - but the CAA at-least declines to offer any meaningful advice on the use of proverse factors.
So, routinely what happens? Where pilots bother to do field performance calculations they may often come up with a field length that is rather longer than the runway available. However, applying some common sense they realise that they've got a reasonable headwind (not included in CAA's factors), a bit of a downslope (ditto), and it's a sub-ISA day (again, not in CAA's factors). So, they know from personal experience that they've got enough runway, and proceed to fly.
Except, that they aren't doing this on any rational basis - they are relying upon "gut feeling". I don't like this, because gut feeling isn't all that reliable where aircraft performance is concerned and sooner or later somebody'll go through a hedge using gut feeling, rather than a meaningful field length calculation (the odds are that they already have).
There's a simple solution to this (in my opinion), which is that documents such as this one (http://www.caa.co.uk/publications/publicationdetails.asp?id=1161) should cover all aspects of factoring light aeroplane performance - including allowing for all factors that improve performance.
I don't see that this should create any unacceptable reduction in safety since there are "mandatory" safety factors applied at the end anyway - but it will at-least mean that performance estimates are reasonably meaningful, and pilots will see some benefit in proper field performance calculations, as well as being equipped to do so.
Thoughts anybody?
G