OM crossing height

[BMM389EC]

May be a silly question but why would both altimeters read 100ft low when crossing the OM than what the published OM height is? Seperate ADCs.

[Tinstaafl]

What was the temperature?

In the absence of any other data I'd first suggest a True Altitude difference.

[BMM389EC]

Aerodrome elevation 5558ft, temp 24 and QNH 1024.

[OzExpat]

It's not unusual to see 100 feet of difference, on the low side. Very common here (tropics). It's not unsafe either because all it means is that your altimeters will show you at DA before you actually get there.

A different story if your altimeters are over-reading though because, by the time they read your DA, you'll actually have busted it by the amount of the over-read.

All else being equal, I like to monitor the Radalt because it catches both situations.

As for the actual reason for the situation... probably something to do with the check altitude being based on a standard ISA day and your conditions not being the same as that. Then maybe factor in a bit of error in the QNH that you've been given. Also maybe - and I mean maybe - factor in a bit of PEC.

[Chimbu chuckles]

Barometric Altimeters are calibrated for ISA, therefore when it's not ISA they will not read accurately.

When it's ISA + they underead which is safe so ignored.

When it's ISA - they overread which is potentially deadly and must be allowed for.

Most carriers/operators who operate in VERY cold climates will have a tabulated correction card to tell the pilot how much the altimeter will be overreading and therefore how much altitude to add to LSALTs/MSAs/MORAs/Minimas etc.

The correction must be added to all heights & altitudes except assigned altitudes (as ATC will have figured in the correction).

A usefull little formula which works to within a few feet until temperatures get bizarre cold and heights above the temperature datum (airfield) get really high is;

+/- 4' per degree of isa devn x (alt/1000)

Note in this formula that alt = height above the datum. The datum is the temperature reference point i.e the airfield.

So you are flying an ILS to a msl desto @ ISA + 20.

ILS published crossing height is 1800'

-( 4 x 20)x (1.8)= 144'

So you would expect the altimeter to read 1656' as you crossed the fix. When it does you can confidantly say "Checked".

At ISA - 20 the altimeter would overead by 144'. But don't just add this to the minima...as some will tell you is mandatory.

If the minima is 250' the math would be + 80 x .250 = +20'. If you then adjusted your minima to 270' when you reached it you would be at the correct DH...and the radalt would confirm that.

As you can see it's not really an issue on precision approaches...which is probably why it is so badly understood...but inbound to a very cold place over high terrain it can get far more interesting.

MSL airport with MSA of 10000', ISA - 20.

+(4x20)x (10) = + 800'.

Obstacle clearance looking a tad sad now isn't it?

Just one good reason to set 250' on the radalt in cruise...and follow ground prox warnings in IMC/night...no matter what you think the altimeter is telling you!!!


Chuck.

[inbalance]

1. some altimeter show a wrong reading when you fly slow, or with flaps. There should be an Altimeter Correction Table in the POH.
For the King Air 200 the Altimeters are 20ft wrong at 110 KIAS and 0 Flaps at sealevel, they are 30ft off with Flaps 40% at 100 KIAS at SL.

Another point is the GS Indicator, it is allowed to deviate 0,2 degree as far as I know. For the OM is usualy 5 NM from the GS Antenna, 0,2 degree gives a Mistake of about 100ft.

sin 0,2 * 5 NM

100 ft Error sounds horrible, but thats the error at 5 NM only, it will become better, the closer you get to the antenna..