altitude

[BigCXJet]

altitude
Can someone answer these, and tell me how to come up with the answer for questions like these?

1) Given an indicated altitude of 10,000, and an actual OAT -20, you set your altimeter to a local station setting of 29.62". If the station elavation is 2,500 ft what is your actual altitude?

2) At a temperture of ISA -25C how much is added to MEA?

Thanks for your help.

[Turbine Cowboy]

BigCXJet,

I think you will find the answear to your question in the ATC chapter of the Jeppessen.
Look up "Flight Procedures"
Part VI Altimeter setting procedures.
Page 244


" Add 30 feet (10 m) to every hPa below STD. to obtain MEA"

When using QNH/QFE however the correction is not required.


Good luck,

TC

[411A]

Least anyone get the wrong idea...absolutely nothing is added or subtracted from the MDA...or DA for the approach, due to low temperatures.
And has always been thus.
MEA's are a different kettle of fish altogether.

[bookworm]

Least anyone get the wrong idea...absolutely nothing is added or subtracted from the MDA...or DA for the approach, due to low temperatures.

411A
Your answer directly contradicts the UK AIP AD 1.1.2 para 3.1.3 which mandates the use temperature error correction for DA/MDA.

[411A]

bookworm,

In the USA (and most developed countries) the QNH given for a station is corrected for local station temperature.

If not done in the UK, then the AIP would apply.

[bookworm]

In the USA (and most developed countries) the QNH given for a station is corrected for local station temperature.

It's not a question of correcting the airport QNH for local temperature. It's about the temperature of the column of air between airport level and DH.

In indicating the altitude at DH, the altimeter assumes that a pressure difference of (say) 7 mbar is equal to 200 ft. With a temperature 30 degC below ISA (i.e. -15 degC), the same pressure difference occurs in 10% less height. So the altimeter overreads by 20 ft.

For typical precision approach DHs, it's hardly worth making a correction. But for an MDH of 1000 ft on a non-precision approach, it would eat 100 ft off your terrain clearance if you didn't make a correction for temperature.

[BOAC]

BIGCX and 411A (are you REALLY a pilot?) should look back in this forum. Many Many threads about altimeter temperature corrections.

Bookworm is absolutely correct.

[m&v]

The Canada Air pilot cold temp' corrections call for a addittion in height at the 200'MDA of about 30-40'at a temp of-30*..Naturally there is no difference to the DH whIch is predicated on the RAD ALT..
As said before I think the introduction of the Alt Setting is a bit o'a red herring.Use the wizz wheel to fine your 'true'Alt minus the Airport alt..:D

'addition'that is:D

10k @-20=9400-2500=6900 AGL ?????:confused:

[Max Angle]

411a, I must disagree with your reasoning about corrected station QNH.

As we all know the altimeter works by sensing the pressure of the atmosphere around it, the instrument is sensitive to changes in this pressure. For it to provide any useful information a datum must be established and this is often QNH but could be airfield elevation (not quite the same thing as QNH), QFE (zero feet indicated at the airfield elevation) or standard setting of 1013.25mb/29.92 inches. Any movement of the instrument up or down away from this datum of course causes the instrument to sense a different pressure. The instrument is calibrated and marked such that a movement upwards of say 1000 real feet produces a similar reading on the instrument face. But lets be clear that all the instrument is sensing is the change in atmospheric pressure over that 1000ft column of air.

The crux of the matter is that the rate the pressure changes over altitude varies depending on the air temperature. The international standard atmosphere model (ISA) gives a standard rate for this change and the altimeter is calibrated so that on an ISA day when it is 1000 actual feet above the datum in use the instrument reads 1000ft. If the temperature is colder than ISA (air is more dense) then the instrument will read 1000ft before it has travelled a 1000ft upwards, ie. it will be reading low. The higher you are above the datum the worse the error becomes. These errors are normally small at the MDA because you are not far above the source datum but even so at -20 degrees C you need to add a 100ft to a 700ft MDA. At 6-7000 feet above an airfield you may have to add 800ft to a published check height to achieve required terrain clearance.

The QNH reading at the airfield may very well be corrected for temperature but that only makes the elevation reading correct on the airfield, if the atmosphere is colder than ISA then corrections must be applied to published altitudes. It's something the Canadians have been doing for years and there is some good stuff on the web about it, have a look at:

http://www.fmcguide.com/media/calgary.pdf

http://www.bluecoat.org/reports/Long_98_Cold.pdf

http://www.atlasaviation.com/feature%20articals/featured-article-2004/lies_your_barometric_altimeter_t.htm

This is something that we have been slow to pick up on in our company, and perhaps in the UK in general but we are now encouraged to think about it in sim. details that include low temps. and I think we are all much more aware of it than we were a few years ago.

Sorry for such a long post but I couldn't seem to explain myself any quicker than that.

Cheers.

PS. I see that several others have posted whilst I was slaving away on this but havn't got time to go through it again to take out any duplication.

[411A]

Max angle,

Very well written post, I must say.
This has never been considered in the USA (to my knowledge) and as most of my flying has been in the much warmer more temperate climates, you may well be correct.
It would seem to me however that with more moderate cold temperatures, the difference is not worth worrying about.

Having said this...do not intend to fly where the temperature is -20C...too darn cold! :uhoh:

[compressor stall]

It certainly has been considered in the USA in this FAA circular (http://www2.faa.gov/language/accold.pdf)

The US has been tardy to implement an official directive re cold weather operations though - as detailed here. (http://www.bluecoat.org/reports/Long_98_Cold.pdf)

You lot in the 48 could do well to listen to your Canadian neighbours.

The question remains if I was your FO, would you have listened to me? :8

[Dan Winterland]

Maybe not in Arizona, but in the cold NE it's known about and currently very relevant. My last two operators applied it when operating in the US - and the USAF certainly do. I would be suprised if others don't.

Worth applying? The error is approx 4' per 1000' per degree of difference from ISA and should be applied below ISA -15. So if you have a MDH (H because it is relevant to your touchdown) 0f 500' and the OAT is -30, you should add 90' to your minima.

Significant I think.

[Field In Sight]

This is how I understand it.

QNH is the QFE (presure measured at the airport) adjusted to sea level using the ISA model.

QFF is the QFE adjusted to sea level using the actual temperature. I have heard that it is used by the met people and also helicopter pilots.

I have a question then.
The DA(H) on an approach is corrected for very low temps.
If you are landing at a very high airport the QFE is still quite accurate whereas the QNH might be very wrong.

Do you still add the same correction to DA and DH???

FIS.

[quid]

FIS-

>>If you are landing at a very high airport<<

If you mean a high "altitude" airport, then the corrections are the same. It makes no difference if the airport is sea level or 5000'. It's the indicated altitude ABOVE the airport that is in error in cold temperatures.

If you mean a high "temperature" airport, then no corrections are made. Your actual (true) altitude will be in error, but you will be higher than indicated. I know of no procedure to correct for this, nor have I ever heard of anyone doing so.

The correction procedure is spelled out very well in Jepp page CA19 or in the AIM (US).

[Dan Winterland]

There's a big lack of understanding flying around here.

The problem arises because the Altimeter is calibrated to ISA. Any deviation from ISA means the altimeter is going to read something other than the true altitude. This is 'temperature error' in the long list of the errors of the altimeter.

If the temp is above ISA, the air is less dense and the altimeter will under-read. In otherwords you will be higher than you think and this is safe. If the temp is below ISA, the air is more dense, the altimeter will over-read and you will be lower than you think. When flying on an instrument approach to a minima carefully calculated relevant to a nearby obstace, this quite clearly isn't safe. Some one has decided that ISA-15 is a good temp to start taking this into account.

The approach minima is referenced to the touchdown point. Whether you start from a sea level reference (QNH) or from the altitude of the touchdown point (QFE) or even with reference to the touchdown altitude with 1013.2mb set (QNE) it's still the gap between you and the ground that's relevant.