What's the Pressure Altitude?

[flyingdan]

Here's the scenario:
@ Muskoka (CYQA), elevation 925' altimeter: 30.85"

The way I calculated the P.A. is:

Code: Select all

P.A. 
= [(29.92 - 30.85 ) x 1000'] + 925' 
= -0.93 x 1000' + 925' 
= -930' + 925' 
= -5'

Is this right?

(a follow-up question is, how would I use this P.A. in reading performance charts?)

Thanks in advance!

[E-Flyer]

When you try to calculate pressure altitude i always like to think 29.92 as a reference.

Ex:
ALT: 4000
ALT SET: 29.80
PA: ?

What do you have to do to 29.80 to get to 29.92? you have to dial UP; how much do you have to dial UP? you have to dial UP .12 inches. Therefore

0.12 x 1000 = +120 (it's positive since we're dialing UP)

4000 + 120 = 4120

PA = 4120


Ex:
ALT: 4000
ALT SET: 30.10
PA:?

What do you have to do now? you have to dial DOWN to 29.92

30.10 - 29.92 = .18

.18 x 1000 = -180 since we have to dial DOWN we get negative.

PA: 3820

[flyingdan]

That's basically how I think about it.

What I was wondering is if my calculations of the PA was correct and how would I use performance charts (i.e. the Cessna takeoff roll charts). Do I use the MSL row?

[AuxBatOn]

You mean Sea Level?

If you use anything HIGHER than what you have (in that case, -35ft), your estimates will be conservative. So if you use 0 ft (ie: sea level), you will be fine.

[Bank Angle]

The answer to your question Flyingdan is YES! Your calculation is right and as far as how to apply it to your performance charts just remember that you CANNOT extrapolate (make up a number outside the tables' limits). So in this case you would use the MSL numbers, which are conservative like AuxBatOn said. Just remember that if you ever find yourself outside the limits of a performance chart where performance is decreasing (ie. the top altitude limit in this case) you have no way of knowing what the performance will actually be and would probably exceed the manufacturer's set/tested limitation of the aircraft if you tried to go flying anyway. Hence the reason we are not allowed to extrapolate.

[trey kule]

I was going to add some comments here, but the bastards that pay me , actually called and want me to fly on a Saturday morning. So will be quick.

Take a few moments to try and figure out how to do all this without all the math. And maybe take a few moments to look at how pressure and temp affect the performance of the plane. With that info, and a knowledge of your plane and the airport involved , you can quickly take a moment to decide if it is really necessary to do any calculations.
The reason for this, is that when you get into the real worlld, you simply are not going to do these calcualtions all the time, so its good to be able to analyze whether its necessary quickly. If your path takes you into the airline worlld, the quick method gives you a moment to confirm the ops/dispatch persons calculations.

Lastly, if it gets to the point that you are on the limits for a take-of for example, dont go. Planes, particularily small general aviaition types, do not usually perform to the specifications in the POH due to normal wear and tear on the engines, gear, wings etc. Anyway thats off the topic, and I have to run