Don't laugh, 

Today I flew KLAX-KDEN. On descending into KDEN I got "Two thousand five hundred" at like 11,000ft

What do I set to overcome this? obviously the airport is at altitude

Set the pressure on your altimeter to QNH or hit the "B" key which will do it for you. 

Also note that the callouts are from the radar altimeter which reads height above ground.

Edited March 23, 20215 yr by cianpars

Nothing unusual I'd say (apart from your barometric altimeter setting possibly); you'd be passing over the Sawatch mountain range as you got near Denver if coming from Los Angeles. This includes many of the highest parts of the Rockies, with Mount Elbert's peak being 14,440 feet above sea level. So if you were anywhere near peaks getting up to 7,500 feet, this could easily make your radar altimeter announce that you were 2,500 feet above the terrain even when you would be up around 10,000 feet above sea level.

2,500 feet is typically the maximum altitude a radar altimeter will read, but since it is part of the ground proximity warning system on every modern airliner, it's certainly going to go off as you fly over some mountains when you're only a couple of thousand feet clear of them. Watch it incidentally if you are using autothrottle, since the GPWS sends data to the autothrottle on many airliners and will command it to throttle back if it thinks you are nearing your landing elevation!

So do I need to know the alt of the airport, if its 3,000ft and I'm at 6,000ft I should assume I'm 3,000 above the ground (without factoring in mountain ranges)

6 hours ago, NZAA said:

So do I need to know the alt of the airport, if its 3,000ft and I'm at 6,000ft I should assume I'm 3,000 above the ground (without factoring in mountain ranges)

Bqasically yes you are correct, although in addition to factoring in mountains, you also need to factor in air pressure settings for your altimeter. Without these, it would be dangerous to merely assume your altitude above mountainous terrain. But ATC and the charts and routes you fly, and the airways and such all factor this in to help you.

Although you probably know most of the following stuff, just to be sure you get it all, here goes...

There are three important Q codes to know where this stuff is concerned (don't worry, it's nothing to do with those conspiracy theory nutters, this is the actual Q Code which dates back to when morse code was common). These are Q.F.E. and Q.N.H. and Q.N.E.

When your aeroplane is on the ground and you turn the dial on the altimeter until the altimeter reads zero, the local air pressure setting you selected to achieve that (visible in the Kollsman window on a traditional altimeter or the Primary Flight Display on an airliner), is Q.F.E. (you can think if it as being an abbreviation for Query Field Elevation if you like, even though it doesn't really stand for that). Q.F.E. is not used as often with airliners as it is with small GA aeroplanes flying in the vicinity of one aerodrome for training and such, so I'm mentioning it here more for completeness, as you will hear it from time to time.

Airliners tend to use Q.N.H. (you can think of this as being short for Query Nautical Height if you like). When your altimeter is set to this value, it would be reading zero if you descended all the way down to the surface of the sea in the local area. Airport charts tell you what height the place is above sea level, so if you looked at a chart for Manchester Airport (EGCC) in the UK, the chart would tell you that the airport is at 256 feet above sea level. But of course for your altimeter which reads the air pressure where you are, you need it set to the local air pressure where the airport is for this to work for you when you land there. So if you got on the radio when you were 50 miles away from there and they told you: 'Manchester QNH is currently 1026.5', you would know that the air pressure at Manchester was a bit thinner and warmer than average, and if you dialled in 1026.5 Mb on your altimeter, when you landed on the runway at Manchester, your altimeter would be reading that you were at 256 feet. Which means that when you were at 1,500 feet on finals to the runway, you'd indeed be 1,500 feet above the ground providing your altimeter was set to that local QNH, but you'd be 1,756 feet above sea level.

But for passing over mountainous terrain, as you were on your flight, you would want to ensure that you were preferably several thousand feet clear of any peaks in the area, and so you could look on charts to see this, but more commonly with airliners, the airline will have planned the routes it flies and have specific rules which you as a pilot for that airline, would tend to adhere to. These would be so far clear of the mountains that even if your altimeter was on the wrong setting, chances are you'd still be well clear of any clouds with granite in the middle of them.

The other altimeter setting you could use - not very common on the ground, but ubiquitous in the air - is Q.N.E. (which if you like can be thought of as being short for Query Nautical Elevation). QNE is often known as the Standard setting on an altimeter and in many airliners there is even a button (amusingly marked STD - no not that STD) which you can press on the EFIS panel which will set the altimeter to this immediately. It is a standardised 'typical' pressure reading which is used for things such as when setting world air speed records and such, as well as being the altimeter setting everyone uses once they pass above the local transition altitude (transition altitude is often 18,000 feet, or 10,000 feet, but in some places in the world it can be a lot lower or occasionally higher than that). Standard/QNE pressure is 1013.2 Mb (if you are in Europe where they use Hectopascals as the pressure scale) or 29.92 Hg (if you are in places like the United States, where they use inches of mercury to determine pressure).

The reason there actually is a transition altitude is that - by way of example - if there were two airfields which were fifty miles away from one another and the pressure at one of them was 970 Millibars (probably raining and horrible), but at the other airfield the pressure was at 1026 Millibars (probably t-shirt weather). If two aeroplanes set off from these airfield and both of them have set the local QFE (current pressure setting to indicate zero feet on the ground) on their altimeters, they are both using different scales, so they will have difficulty maintaining vertical separation from one another if they were to meet up. But if after they take off and pass above a transition altitude, they would both put their altimeters on the Standard setting, when they got near to one another, their altimeters would be using identical scales, so if one of them radioded that he was at 15,000 feet and the other replied that she was at 16,000 feet, they would know there was absolutely no danger of them colliding, since they'd definitely be 1,000 feet apart vertically even if they both passed over the exact same spot on the ground.

Of course the air pressure changes all the time, so airports have an ATIS (Air Traffic Information Service) radio frequency, and you can tune into this and hear a recorded message which loops. This is updated every hour and uses a sequentially advancing phonetic letter to identify which hour it is from, so that when you tell ATC what information you have, they will know whether you have the current info (unless there is a significant change before the next hour's recording is due, in which case it might change sooner).  The ATIS message will tell you stuff such as wind, temperature and the local air pressure (QNH), so that you could set this and have your altimeter read the elevation of the airfield above sea level when on the ground there.

Hope some of that helps. It's not as complicated as it seems and in your simulated airliner, your radar altimeter and terrain collision avoidance system would help you a lot!

 

Edited March 24, 20215 yr by Chock