View Full Version : Light aeroplane performance - use of proverse factors

Genghis the Engineer
5th Oct 2004, 14:58
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?


5th Oct 2004, 16:09
I think the CAA are covering their backside slightly. They offer us what would happen in the worst case scenario, and if you're still good to go then fine, go.

However they may think that some pilots would become over confident with proverse factors and do precise calculations if they're in a tight situation, which they then work out wrongly, and get stuck in a hedge.

There have been a few times when officially i shouldn't have gone, but i knew the a/c and the conditions were favourable and made it easily. Then again there are also times when according to the book i could go, but i knew the engine(s) were old and the a/c was a dog to fly so i jacked it in for that day.

I agree that proverse factors can be useful, but then there needs to be a simple way to work them into the other calculations (remember we're only pilots, not mathematicians:E ) i still remeber K.I.S.S. from flight school.

5th Oct 2004, 17:56
With regard to the increase in distances required for the CAA's "10% in crease in weight e.g. an extra passenger".........the intention was never to suggest that the take-off weight ever could be above MTOW...........that factor was designed to cater for aircraft such as the C172 which published performance figures for weights below MTOW and the CAA were trying to provide a simple factor to use if a last minute passenger arrives who will put the take-off weight above the planned one but not above the MTOW.

Many "light aircraft" such as the pipers have graphs that permit allowances for headwind, down slope, better than ISA, lower than max weight etc.

However, if you are taking the professional side of aviation as a model of safety, remember that we have to demonstrate that we can operate from the longest runway at an aerodrome in zero wind and from any other runway with an allowance for any headwind.................since most light aircraft operate from airfields where there is often only a single strip, then it must be capable of using that strip in zero wind.

When using wind for performance, only 50 percent of a reported headwind can be used........again many light aircraft flow privately give up when the wind is above 20Kt thus the best ever wind to be used in the calculations would be a 10Kt headwind.

How many small airfields used by light aircraft have accurate wind reporting, calibrated pressure reporting and temperature reporting and have had a survey showing exactly what the slopes are? The fields with all those tend to be plenty long for light aircraft!

The CAA factors ensure that take-off, never requires more than 75 percent of the runway available and landing never more than 70 percent........which may leave observers thinking that "hey, I can get away with a lot shorter than this".........yes one can but the safety margins are reduced.

I know some aircraft that I can land in under the book distance most days and match it on the rest..........but I know plenty of other pilots who fly the same aircraft and require more than the book distance because of lack of practice or the old adding on a few knots problem.

Any pilot who comes to me and says that he can match the book figures gets the chance to demonstrate their ability and few ever manage to repeat the performance consistently during a few circuits at a couple of aerodromes over an hour or so.

Just some thoughts.