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Moderator: AndyR

By archerflyer
FLYER Club Member  FLYER Club Member
#1904804
Hi All,

sorry if this is a daft question & i've messed up a topic, but I'm doing some take off/landing distance prep and i've come across a scenario I haven't encountered before and not sure on the next steps - thought I'd ask here.

Using the table from the POH for calculating take off and landing distance, what do I do in the event that the pressure altitude would be below sea level?

example:

airfield elevation - 390ft
QNH - 1028
1028 -1013 = 15 *30 =450
390-450 = -60FT

Would I use SL data?

thanks in advance and help much appreciated.
By blackjack_201
#1904828
Using SL data is safest as it is more conservative. And other alternatives might be too much work for what is a small difference relative to SL (and again, with using SL giving you the safe option).

If I was desperate for the -60ft:

Assuming the lines for the other pressure altitudes are parallel and are equally spaced, then you could assume a graph line parallel to the others and at the same spacing as the others and interpolate the results for -60ft.

The other option could be to use the CAA factors for the change of distance per 1000ft change in altitude but instead of adding 10/5% to your TO/landing distances you subtract them. You'd have to scale the 10/5% for 60ft (rather than 1000), which would give a very small delta to SL.

But again SL data is more conservative hence safest. And if 60ft of altitude means the difference between you going to this place or not, I'd argue it's probably best to pick another place to go :)