Question about plane altitudes and high-altitude cities.

Factual Questions
1

The last time i flew back to Baltimore from San Francisco, i came through Denver. Whenever i fly United, i like to listen to the air traffic conversations (usually on channel 9 on the in-flight system), and as we approached Denver i heard the pilots and the controllers sounding off altitudes, as they usually do, like “United 973, descend and maintain flight level one three zero.”

The thing is, though, that Denver, the “mile-high city,” is actually about 5000 feet above sea level. So, when my plane flew out of San Francisco and reached 5000 feet, giving a beautiful view of the bay and the ocean, it was still only at ground level, if measured from Denver.

What i want to know is, how do they describe altitude from high cities like Denver? Do they always talk about feet above sea level, or do they talk about altitude above the ground?

I’m assuming the former, but that would also require some thinking on the part of the pilot. After all, if you descend to 6,000 feet near San Francisco, you’ve still got plenty of wiggle room. If you do it at Denver, your ground proximity alert will be going off pretty quickly.

And, if the latter—if they measure altitude from the ground—where is the changeover place. That is, where, between SF and Denver, does the shift happen?

What sort of procedure do pilots and controllers use for taking into account the altitude discrepancies between different airports.

2

Aircraft altitudes are discribed in two ways:

Feet above mean sea level, and flight levels. Flight levels start at 18, which is nominally 18,000’ MSL. I say nominally, because flightl levels are always based on a “standard” barometric pressure of 29.92"Hg regardless of weather conditions. “feet” on the other hand uses a basic correction for barometric pressure, roughly 1000’ per "Hg. The ability to make this adjustment is built into all aircraft altimiters. The basic design is known as a Kollsman altimeter, and the barometric pressure setting is displayed in the Kollsman window.

Aircraft flying long routes might risk collision if they kept the baro setting from thier departure airport. Since everyone uses the same baro setting above FL18, seperation is assured.

On the other hand, when landing or operating near the ground, real, honest altitude is needed to avoid violent, unplanned landings. Hence the two systems.

Note that barometric pressure is NOT the air pressure at ground level, exept at sea level. The actual air pressure measured in Denver is adjusted upward by about 5" to obtain barometric pressure.

Below flight level 18, Aircraft altiudes are always referenced to mean sea level. Upon decending below Flight level 18 the pilot will adjust the altimeter to the local barograph setting. At small, but controlled, airports, the controller will pass this information to the pilot “Altimeter 27.53” for example.

But you won’t hear this on an airliner. At large airports, the baro setting is passed via an automated system (ATIS, airport terminal information service, I think) that plays a loop of all the information that every pilot should know. This is periodically updated, and given a keyord (always a standard phonetic letter)so that the controller can verify that the pilot has the current information. Thus a pilot might check into the traffic pattern “Houstin approach, United 635 with information Uniform”

You may have deduced that near 18,000 feet, or FL 18, the two systems may produce some overlap. This is true, and thus it is rare to assign an aircraft to an altitude above 17,500’ or below flight level FL18.5.

3

Doh!

ATIS = Automated Terminal Information Service

4

Pilots need to reference absolute altitude for other reasons as well. Take-off from high altitude airports has very real effects on the plane. The plane doesn’t really care that you are only 100 feet above the runway at an airport 6000 feet above sea-level. The plane will still behave like it is 6000 feet above Kansas in terms of both engine and aerodynamic performance. For that reason, there is a calculation “density altitude” that tells the pilot the aerodynamic altitude that the plane will feel even during the take-off roll. It isn’t just based on altitude. Temperature also plays a roll. High altitude and high temperature both decrease air density and decrease the lift of the plane as well as engine performance on non-turbocharged piston engines typically found on small planes.

The effect can be dramatic at a high-altitude airport on a hot day. The plane may not be able to carry a normal payload or the runway may not even be long enough to take-off at all based on the conditions.

5

I don’t know if we were clear above for the simple version of your question but absolute altitude above sea-level is the important number.

6

That’s actually flight level 180 (which it seems is always spoken as “one eight zero”, never “one hundred eighty”). Here’s a link.

7

Thanks for the responses folks. Very informative, and it all makes good sense.

8

Actually what you are more likely looking for is the difference between MSL (Mean Sea Level) and AGL (Above Ground Level) altitudes.

As has been stated the Flight Levels are reserved for higher altitudes (in the US 18,000 feet is the changeover but it can be any altitude - several countries in Europe use 6000 feet as the changeover). One thing - in the US the controller is required to give you the local altimeter setting when you are first cleared below FL 180.

For radio transmissions between controllers and aircraft MSL is used. For example when flying into Denver you might be given a descent to 7,000 feet. Yes you are only 1800 feet AGL, but MSL is referenced. The controllers have Minimum Vectoring Altitudes (MVAs) that they use to keep airplanes away from high terrain. The simplified altitudes given in clearances (MSL) help the controllers keep airplanes away from each other.

The only time AGL is really referenced is on instrument approaches, and then it is only on the flight deck (ie we talk about it in the briefing but don’t say anything on the radio about it).