DENSITY ALT CONFUSION

[JIMBO01]

Can someone help me. I'm completely confused about density altitude.

I flew an aircraft, I was at a pressure altitude (1013mb) of 15000ft. It was ISA+13 which was indicating a density altitude of about 16800ft. My question:

1/ To obtain power figures and climb rates, my AOM gives tables of pressure altitudes only to derive RPM, TAS, and GPH at ISA -30, ISA and ISA +30. Do I use the density altitude (16800ft) to be the 17000ft pressure altitude figures in the table of the AOM or do I use the 15000ft pressure altitude im currently indicating?

2/ If my aircraft maximum operating altitude is 17000ft and I'm at 15000ft but my density altitude is giving 18000ft, am I now exceeding the aircraft maximum altitude capability?

Basically, does density altitude affect the maximum cruising level I can go to when my aircraft is certified to fly at a given max operating altitude?

[wiedehopf]

1: you could use the density altitude with the numbers for ISA, but because your engines are getting hotter air than anticipated by the person who made the table, this is probably a bad idea.
to be on the safe side just use the pressure altitude and ISA+30

2: well if numbers for ISA+30 at 15000ft pressure altitude are in the table then ISA+13 at 15000ft should not be a problem should it?

[Vessbot]

1. The chart is asking you for pressure altitude, so you use pressure altitude.

DA is PA adjusted for temp, right? Well in using the chart, you're picking a temp column, so you're taking temp into account. If you started with DA, you'd be taking it into account twice. Double dipping, as it were.

[john_tullamarine]

Unless the manual says to do something else, use PH and interpolate for that actual OAT using the manual deviation data

am I now exceeding the aircraft maximum altitude capability?

I think all is well .. the AFM/POH will give a maximum PH and ALSO a maximum ISA deviation.

[pattern_is_full]

1. For aircraft performance, use the DA. If the chart doesn't have a column close to your DA - interpolate (real temp ISA+13 = about halfway between the settings for ISA and ISA+30)

2. Depends somewhat on what it is that limits you to 17000 feet MOA. Pressurization (differential not sufficient? Or no pressurization, just nasal oxygen?) Structural (too high a pressure differential?) Engine? Equipment (insufficient for USA class A airspace? Not RVSM-compliant? etc.)

Or was 17000 just a number you picked as an example?

As Vessbot says, the definition of DA is "PA corrected for temperature" - so I'd take DA as your "effective PA" and stay below 17000 DA, unless I had clear evidence from the POH otherwise.

[ACMS]

John Tullamarine and Vessbot are correct.