Deepest Norfolk
28th Feb 2013, 21:48
Looking at a photo of an Azul E190 departing Santos Dumont, there looks to be an awful lot of flap. Just wondered, with such a short runway is the E190/E195 WAT limited out of there?
Thanks.
john_tullamarine
1st Mar 2013, 00:26
Can't speak to the Embraer AFM details but your question highlights a general problem.
WAT limits are quite independent of aerodromes, runways, and the temperature of your cup of coffee.
Looking at establishing RTOWs, the limits generally are considered in something like the following order to find the limiting (ie lowest weight) case for each RTOW point -
(a) max certificated take off weight. This is per the TC absolute max takeoff weight, regardless of any other considerations. (An individual aircraft may have a locally varied figure for whatever reason - usually lower). This limit often is based on structural considerations of one sort or another.
(b) WAT limits. At different conditions of (pressure) Altitude and (outside air) Temperature, the maximum Weight may need to be reduced below the TC/AFM certificated limit to ensure that the aircraft has a chance, OEI, to establish at least a modest rate of climb, even it that needs a bit of help from the curvature of the earth ... There is no point taking off at the TC limit on a hot day from a high elevation runway, have a failure, and then collapse back onto the runway overrun area. The various WAT limits (which may be presented in the AFM as a combined chart) look at the various takeoff phases, etc.
(c) runway limited weights
.. and a few other items to consider as well.
If one has a very short runway, then that is going to predispose towards a high flap setting. The down side is that the WAT limited weight at the conditions AND for that larger flap setting will be LESS than what would apply for a lower flap setting. So, for a given runway on the day, it's a case of juggling the WAT limit variation with flap, and the distance limited weight for the runway variation with flap .. to end up with the highest weight for which both the WAT and runway distance limited weights are acceptable.
Just which limit becomes the limiting case would require an analysis .. ie, one cannot generalise on this sort of thing.