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I needed clarification for one question I've had for some time now.
The question was for take-off chart "surfaces";
1st thing I have is when it's stated "unrated gravel/grass, I'll take the short wet/reference line? and secondly if it said "gravel" I'll take long wet grass?
Thank you for clarity ,
Another question arises which is when looked in the ERSA on "surface" c or C = other surfaces (always to be qualified by a note ); my question is what if no specifications are given, will you take the reference line as your surface on the take/off chart?
You will always get information note what type of surface is C.
However, in real life I would walk the air strip and check it myself, since the grass may not be mown, or how wet is wet. I would also calculate the distance in worse case scenario (long wet grass) and confirm that I have enough runway to take off..
We need to keep in mind that the AIP relates to all aircraft and its primary interest tends more to heavy aircraft rather than the light GA fraternity. In addition to AIP material, you may also like to read through MOS 139 relating to aerodrome standards.
Specifically, in the aerodromes material, the light aircraft can operate from any suitable surface.
Traditionally, we have followed the protocols -
(a) takeoff and landing performance flight test work routinely is done either on short, dry grass or seal surfaces. (As a side note, I have a very old 748 OEM test report for some trials from a real mudpit runway and, boy, were some of the photographs of the ruts created eyebrow raising).
(b) typically, for heavy aircraft, gravel is penalised 15% compared to seal, although we also used short dry grass figures as being OK for gravel for light aircraft.
(c) for other surfaces (SWG, LDG, LWG) the figures have nothing to do, really, with those surfaces. On the basis of various friction coefficient testing, a range of coefficients reasonably representative of these surfaces was established and forms the basis of the DCA P-Chart graphs. There is no point worrying about the actual friction numbers as they will be pretty meaningless to most folks.
There is no simple quantitative way the pilot in the field can fiddle the numbers to provide more accurate data on the day. However, it is very sensible, should the charts suggest you are close to being limited, to err on the side of caution in the selection of friction coefficient lines.
The old stock standard ag pilot approach of running a takeoff empty to see what the aircraft actually does, and then increasing the gross weight cautiously to arrive at a comfortable figure, has much to recommend it. Certainly, for glider outfield retrieves in the dim past, and other operations from tight paddocks, this was an often used technique by this pilot.
Bob's suggestion, as noted by Bosi, is quite reasonable for most situations.
For the exams, however, things tend to be a bit black and white so use whatever surface the examiner indicates. If there be no guidance, then usually short dry grass will be the way to go.
Engineering specialist in aircraft performance and weight control.