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How do you determine pressure altitude at an LZ before flight?


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Hi,

 

I know HIGE and HOGE charts require aircraft weight, temperature, and pressure altitude to calculate a ceiling.

 

I know pressure altitude is read off of the altimeter when it is set to 29.92, but when I'm flying a helicopter, I can set the altimeter to 29.92 and read the pressure alt, but I can't consult the charts while flying.

 

Does anyone have any tips on doing this on the ground before a flight? Do I just use the standard rule of thumb for temperature, ie. 2 deg C per 1000' and for pressure altitude, calculate the pressure altitude at my departure point and then subtract a 1 inch per 1000' to come up with a figure for an LZ at a higher altitude?

 

Does anyone have any tips or tricks for this?

 

Thank you

 

 

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I had to find the pressure altitude at an off airport LZ as part of the performance planning part of my commercial checkride.

 

Just subtract 29.92 from the current setting, multiply that by 1000, and add it to the LZ elevation. That should give you the pressure altitude at your LZ,...I think, it has been a while?

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Flying into an environment that will limit power (hot and high) I work a graph of limits from RFM graphs, etc., of temps and altitudes where the HOGE weight starts falling. You need HOGE for most helo operations...

Example, a grid with 3500' PA and 40 deg. C allows book max gross; 4000' and 35C; 4500' and 30C. Estimate altitude at the LZ (or use the altimeter PA as described), note the OAT., adjust for surface heat, and interpolate between the points. I chart this for every 500' PA in conditions I'm likely to encounter.I won't be higher than 5500 in my service area, and I'll never be lighter than "X" at the LZ, so I don't figure for improbables. This grid is a quick reference at dispatch/departure and prelanding.

Then, do a power check! and use whatever rule of thumb necessary to check your calculation- 1% NG/3% TQ is 200 lbs, for instance...

 

The lapse rate is for free air, and local conditions are whatever exist where the aircraft actually is. Local, surface heating will change density altitudes faster than predicted lapses. The last thing you want to do is be stuck where you can't depart, so leave yourself some margin until you know how much slack you need.

Edited by Wally
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You can't know the actual DA without knowing the actual temp and humidity, and that changes constantly. All you can do is use the best information you have available, and give yourself some margin. You can use standard lapse rates if that's all you have, but again, leave some margin because actual conditions will almost certainly be non-standard, one way or another.

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Pressure altitude, or altitude, changes constantly. Just use the elevation of the LZ as it appears on a map. If a little fuzziness on exact altitude matters that much, it means you don't have enough power reserve. You want to have too much power; not just enough. Go with the documented LZ elevation.

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Pressure altitude, or altitude, changes constantly. Just use the elevation of the LZ as it appears on a map. If a little fuzziness on exact altitude matters that much, it means you don't have enough power reserve. You want to have too much power; not just enough. Go with the documented LZ elevation.

 

That's my take away...

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  • 2 weeks later...

Pressure altitude, or altitude, changes constantly. Just use the elevation of the LZ as it appears on a map. If a little fuzziness on exact altitude matters that much, it means you don't have enough power reserve. You want to have too much power; not just enough. Go with the documented LZ elevation.

+1

 

Any time you are going to need to "thread the needle" in order to pull something off, you should just stay on the ground.

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