METARS & TAFS
Oct 18, 2017

Airspace
Jul 26, 2017

Altimetry
Jun 19, 2017

Airports and Circuits
Apr 11, 2017

Whiz Wheel Time
Feb 16, 2017


Altimetry
2017-06-19
A) How does an altimeter work?


The altimeter is vented to the atmosphere through the static port. Inside the altimeter is an aneroid capsule which contracts and expands as the atmospheric pressure changes, either by you climbing or descending or by a change in the current atmospheric pressure.


 

B) How do you calculate your Pressure Altitude?


If your flying it's easy. Just turn your altimeter sub-scale to 29.92" of Mercury and your current Pressure Altitude can be read off the altimeter. If your on the ground a little bit of math is needed.

First you need to know the current altimeter setting which you can get off the airport METAR. Now you need your International Standard Atmosphere pressure. Remember what that is? 29.92" of Mercury.

Now minus the current setting from the standard and see what you get. If the number is negative then minus that height from your elevation, if it's positive add it. For example:

Airport Elevation 2567 ft
Airport Altimeter 30.17
ISA Altimeter 29.92

29.92 - 30.17 = -0.25

Now it's just a case of getting the decimals in the right place. The far right digit is worth 10 feet the next 100 feet and the left of the decimal would be 1000 feet. It came out negative so we're going to minus

Our pressure altitude = 2567 - 250 = 2317 feet.
 

C) Will your altimeter over or under read if the pressure is dropping and you haven't adjusted the sub-scale?


"From High to Low look out below"

With out any adjustments to your altimeter it will be over reading. For every 0.10" inches of mercury pressure change your altimeter will be wrong by 100 ft. For example:

Your altimeter on departure was 29.80". After flying for the afternoon you return to the airfield without adjusting to the new altimeter of 29.20". A difference of 0.60 or 600ft.
 

D) Your working in the Northern Domestic Airspace (NDA) 15nm from an airfield. What would your altimeter be set to?


When flying in the NDA your altimeter is always set to 29.92" of Mercury unless you are landing or taking off, in which case you would use the airfield altimeter setting.

See your AIM Section 2.10 for more information
 

E) What is Density Altitude?


It's pressure altitude corrected for a non standard temperature. Its a good indication of how your helicopter will perform. If your density altitude works out to be 5000 ft your helicopter will perform as if you were flying 5000 ft above sea level on a standard day. To calculate your density altitude either use your trusty old whizz wheel or the chart in the performance section of you Helicopter Flight Manual.