Altimetry - how do you teach it in the US

Ozgrade2

Newbie
Sep 25, 2002
5
0
Hi all,
I was wondering how you guys in the US teach altimetry, teaching students to work out preasure height and density height.
Here in Oz, we use metric (deg C and pressure in hectopascles). If the QNH is 1003Hp, we take the difference between that and the standard ISA QNH of 1013 Hp =10 hp, multiply that by 30 ft per Hp that gives us the PH. 10 x 30 = 300. Therefore PH for that day at msl is + 300 ft.if the temp was 25 deg c, its''s ISA + 10 (ISA being 15 deg C at msl) 120 ft per degree from ISA =1200 + 300 PH =1500 density height.
How do you guys work this out using inches of mercury and deg F.
 

kcabpilot

Senior
Aug 22, 2002
271
0
Pretty much the same only the units are different. Standard is 29.92 inches of mercury and 59 degrees F which is equal to 1013 hPas and 15 deg C. In general an inch of Mercury equals 1000 feet which (rough guess) would equal about 33 or 34 hPas?

Now my question is: if you Oz folks are metric, why are you using feet?
 
OP
O

Ozgrade2

Newbie
Sep 25, 2002
5
0
Hi mate,

Thanks for that, can you work out a problem, till cant get my head around it using inches and deg F.

To make matters worse here, we use feet for altitudes and distances vertically from clouds, typically 500 feet vertically from clouds, but must maintain 1,500m horizontal distance. Runway distances are in metres but track distances are in nautical miles. Weights are in KG.

We've been metric for over 20 years, but most peole still relate peoples height in feet an inches. Even the young kids do it, when you say soeone is 5'7 I know exactly how tall they arem, but expressed as 172 cm leaves me with no clue.Yet, I only know weights for people in KG, I'm 94 kg. Go gigure.

I think I'll have an asprin and a good lay down *LOL*

cheers.
 

kcabpilot

Senior
Aug 22, 2002
271
0
Yup, it was about 20 years ago that our Government, in all their wisdom, decided that we were going to convert to the metric system. The first up were gas stations that started selling gas in liters. Only problem is that the first ones to convert saw all their customers across the street buying gas at the station that still had pumps with gallon units on them. There was a time when many had pumps that displayed both liters and gallons but eventually they all gave up and now days you won't see a single pump anywhere that has anything to do with liters. Same thing with meters/feet, pounds/kilograms etc. The end result is a real mess. There are even cars that have half metric, half SAE stuff on them. Everybody has to have two sets of tools.

As for the density altitude problem well normally you just take out your trusty old E6 flight computer and line up your pressure altitude with the temperature, which - go figure - is in degrees Centigrade, and read your density altitude next to the little arrow labeled such.