Wind and Altimeter Setting

[WestBoundPilot]

I've been told that if, on a long cruise, the plane drifts left (port) then the altimeter will read too low. For the life of me I can't work out why that is the case. I thought that if I was drifting left the plane would be moving towards a low pressure system (or away from a high pressure system) since in high pressure systems the wind blows outwards, and inwards in low pressures systems.

I would be grateful if anyone could explain what I've got wrong. Thanks!

[Piltdown Man]

In the northern hemisphere with your back to the wind the low pressure will be on your left.

PM

[eckhard]

In the northern hemisphere, port drift means you’re flying towards higher pressure, so the altimeter will under-read and your true altitude will be higher than indicated. Starboard drift means you’re flying towards low pressure so the altimeter will over-read which could be dangerous.

In the Southern Hemisphere it’s the other way around.

The reason is to do with the direction of air circulation around a pressure centre.

In the northern hemisphere, the circulation is anti-clockwise around a low and clockwise around a high.
Vice-versa in the Southern Hemisphere.

If you draw a few diagrams you’ll soon get the idea.

[WestBoundPilot]

In the northern hemisphere, port drift means you’re flying towards higher pressure, so the altimeter will under-read and your true altitude will be higher than indicated. Starboard drift means you’re flying towards low pressure so the altimeter will over-read which could be dangerous.

In the Southern Hemisphere it’s the other way around.

The reason is to do with the direction of air circulation around a pressure centre.

In the northern hemisphere, the circulation is anti-clockwise around a low and clockwise around a high.
Vice-versa in the Southern Hemisphere.

If you draw a few diagrams you’ll soon get the idea.

Thanks eckhard, that makes a lot of sense now! I was thinking of air flow out and in rather than clockwise and anti-clockwise. I drew a few diagrams and it all clicked.

[Ascend Charlie]

It won't matter a rat's patootie, because any other aircraft in your area will be on the same QNH so your relative altitudes are the same.

If you are flying so low to the ground that a 30' difference between indicated and real altitude means you will hit a hill, what the heck are you doing, flying IMC without being at LSALT, or VMC and not looking outside??

[eckhard]

Love the ‘patootie’ reference!

I think that the OP’s question falls into the same category as the one about flying from Bombay to New York without re-setting the DI. What will be the indicated heading after 14 hours? Who cares?

Still, it might be useful to be aware that in a remote northern area, without an updated QNH and experiencing continuous significant stbd drift, that the aircraft will be descending with a constant indicated altitude.

I wish that your comment about “all aircraft being on the same QNH” was true in the UK! Unfortunately, what with low Trans Alts, Regional Pressure Settings, QFE and unregulated IFR being possible in Class G, there is a real possibility of two aircraft at the same level having different settings......