View Full Version : Over-reading/Underreading Altimeter - Question

23rd Jun 2002, 18:38

Hope you can help me with a little problem I'm having with a question in PJ Swatton's Performance Theory book.

En-route in the Northern Hemisphere an aeroplane flying with QNH set on the sub-scale of the altimeter, is experiancing 15 degrees port drift. The altimeter, if the subscale is not reset will:
(a) Read the correct height above MSL.
(b) Over-read
(c) Under-read
(d) Not be affected.

The answer given is (C) - Under-read. Why?

I've trawlled through several books/notes (Performance/Principles of Flight/Atmophere etc) and I can't find any relationship to drift/hemisphere affecting altimeter reading.

It's really bugging me.

Glad of any help you guys might be able to offer.


Hew Jampton
23rd Jun 2002, 19:02
Because the aircraft is experiencing port drift, it is heading into an area of higher pressure (wind blows clockwise round an anti-cyclone in the Northern Hemisphere). If going into higher pressure, the altimeter will under-read, ie the aircraft will be higher than indicated by the altimeter. This is safer than the converse: when entering a lower pressure area, the altimeter will over-read, ie the aircraft will be lower than indicated, or 'When flying from high to low, look out below'.

oxford blue
24th Jun 2002, 22:17
Remember Buys Ballot's law? Stand with your back to the wind, low pressure is on your left.

OK, so you're experiencing port drift. Port drift means that, if you turn to the left, you'll be standing with your back to the wind. So low pressure must be to your left (ie, behind of you before you turned to the left).

So you're going from a lower pressure situation to a higher pressure situation.


or, conversely:


Going from a LOwer pressure to a HIgher presure situation, your altimeter will read LOw. Ther aircraft will be higher than the altimeter says it is. This is the safe case.

Therefore the altimeter will be under-reading. The aircraft is actually higher.

Your mistake is to treat Performance as a self-contained study. The JAA exam system encourages you to think like that, and it isn't helpful. There aren't 14 different subjects; there's only one. It's called Flying and Airmanship. It just happens to be split into 14 different exams.

When I'm feeling expansive, I emphasise this point by telling my students how I once discovered a pitot leak by noting that the fuel-flows seemed a bit high. They all say "Surely not, fuel flows are Performance and Gas Turbines and pitot leaks are Instruments. How could you possibly have diagnosed a pitot problem from fuel-flows?" But I did, once. I'll tell you how, if you're interested.

It's all one subject.

25th Jun 2002, 02:03
Poor SAR huh? ;)

26th Jun 2002, 19:19
Many Thanks for taking the time to respond to my question.

I fully understand what Oxford Blue is saying. Somewhere amoungt these trees is a forest. :rolleyes:

I haven't covered "Met" type things since my PPL (about 10 years ago) and I am sadly lacking any knowledge in that area, so I struggled to see the whole picture with respect to the question. I knew there was something relevant - just didn't know what!

Will always keep in mind that its one subject.

Checkboard - I don't follow your comment?

[Edit]Oh and Oxford Blue - yes I would be interested....[Edit]

26th Jun 2002, 20:40
Checkers is referring to Specific Air Range ie the ratio of TAS to Fuel Flow.

If the pitot/static system has an error then the corresponding TAS will be in error, resulting in an odd SAR

oxford blue
26th Jun 2002, 20:46
Checkboard's is right, but I was too inexperienced to check that at the time. It's not a normal pilot or navigator check. It Checkboard a flight engineer, by any chance?

Bumpfich, I'm so pleased that you would like to hear the story. I thought no one would ever ask!!

I was in a Canberra and we noticed that the fuel flows seemed a bit too high. We were flying the right indicated Mach No. I did note that the wind component seemed a bit lower than forecast, but I thought nothing of it. Met forecasts aren't always that accurate.

When we landed, I reported to the ground crew that the fuel flows seemed a bit higher than usual. No big deal. On the Avon, it was usually the P1/P3 switch. They said they'd have a look at it.

We had to fly the same aircraft the next day and when we signed for it it, we noted that the engines had been test-run and 'no fault found'. We did rather feel that the ground crew gave us a bit of an old-fashioned look that implied "I've been up half the night running bloody engines which were perfectly serviceable because the aircrew can't diagnose the fault properly", but we put it behind us and got airborne.

Exactly the same thing happpened on that trip as well, except that this time the headwind component was even lower than forecast than before. Fuel flows were high, just as before. This time the controls felt a bit stiffer than usual, too. And then it struck me! We knew what the aircraft felt like when you flew it at M.75 instead of .73, because we practiced it occasionally. And it felt just like that.

OK, so that must be the problem. The machmeter must be under-reading. That would explain why the W/Vs seemed wrong. You calculate your TAS from the machmeter - or ASI- it'll give the same result. If we used .73 instead of .75 to calculate a TAS, we would be calculating too low a value. Then when we compared it with the groundspeed from Doppler or from a succession of fixes, the headwind component would be too low. It all fitted. But it's the one fault where there is no independent confirmation. There is only one source of TAS - the pitot static system - whether you use CAS or Mach No to find it - you are still using a comparison of pitot and static.

So how to check it? You can't trust met forecasts - at least, not that accurately. And then it occurred to me. Our route took us back to base on the reciprocal of the outbound track. So I could get a groundspeed outbound (from the Doppler) and a groundspeed on the return). If we held the same Mach No and the wind hadn't changed, the TAS must be the average of the outbound and the return groundspeeds. So we tried it and - joy of joy! - we appeared to have a tailwind component in both directions! Obviously, it was our calculation of TAS which was out.

When we landed, I asked the groundcrew to have a look at the pitot-static system this time. We got a phone call about an hour later saying that they'd discovered a HUGE pitot leak.

That explained it all! The reason that the fuel-flows were too high was because we had set the power to fly at .75 instead of .73. Nobody really notices what the required rpm is - you just set whatever rpm you need to get the speed you need. Our TAS was higher - but we couldn't tell. because there is no independent check - and our Doppler groundspeeds were right. So we had been finding the wind and had found too low an apparent head-wind component, but- hey - so what?

Effectively, we had diagnosed a pitot leak from a fuel-flow indication. As I said earlier, it's all one subject!

Dick Whittingham
29th Jun 2002, 19:09
Great story, Blue, and I'm sure you and Hew won't mind me pointing out that your answers were for low level flight, where barometric error is dominant. If you are holding a constant indicated altitude, you are flying on a pressure level, and the height of the pressure level amsl is determined by a combination of msl pressure and the thickness of the layer of air below you. This, in turn, is determined by the air mass temperature. At height it is still true that if you have port drift in the N hemisphere your true altitude is increasing, but you don't know whether it is caused by baro error or temperature error or both.

Agreed, at FL300 obstacle clearance doesn' matter much!

Dick W

4th Jul 2002, 07:31
Oxford blue, very impressive reasoning. You mentioned Avon, Comet or Canberra?

oxford blue
4th Jul 2002, 08:38