Ask Captain Lim

Hi Captain Lim,

Thanks for an extremely informative site. I fly 50-60,000 miles a year - mostly international - so I enjoy very much, learning more about my mode of transport.

Regarding a previous FAQ - In trying to explain the difference between air speed and ground speed you might use the example of the ubiquitous moving walkways that we find in nearly every major airport today. I can be walking at 2 kph on the floor but that 2 kph coupled with the moving walkway speed of 1.5 kph puts me covering 3.5 kph even though I*m walking at the same speed.

Now for a question: I'm writing from Denver, Colorado where our airport sits at 5200 feet above sea level. We have very few international flights here and while one reason is simply demand, another explanation has to do with elevation, and range of the aircraft relative to flying out of Denver. I understand that Mexico City at 7000+ feet of elevation has a similar problem.

When I have probed into it, I've been told that the main problem is the amount of fuel that can be loaded and that in turn limits range which in turn limits the potential flight offerings. But wait, you would think the thinner air and higher elevation would be a help rather than a hindrance. I hope this question makes sense.

Thanks,

Chris

P.S. Just a neat piece of trivia; did you know that pre-production testing of the earliest jet engines were conducted near Denver on top of the world famous Pike's Peak (14,200 ft) to see if they would work at higher elevations as we now know they in fact often work better in the thinner air but all those years ago it was proven here in Colorado.

Hi Chris,

Nope! Thinner air and higher elevation is not an advantage to airplanes. Why? Well, as you would have realized, the density of the air decreases as one climbs above the mean sea level. Lower air density affects a plane in three ways: the lift produced by the wings, the power generated by the engines and the thrusts of the jet engines. Yes, you may argue that a jet engine is more efficient at higher altitude ? ah, the total performance losses are much more in relation to any reduction in drag or increase in fuel efficiency!

In practical terms, how would it penalize a commercial airplane? Well, the airport must have longer runways than those situated closer to the sea level to cater for take offs and landings. Payload may be reduced and this can also affect the amount of fuel carried on board airplanes. For instance, on the extended-range Boeing 777, to fly an international non-stop flight of 12 hours, it needs to load up to about 100 tons of fuel, or about 65 to 70 percent of the basic weight of the plane.

Yes, Denver, USA (5280 feet), Johannesburg Airport, South Africa (5558 feet) and Mexico City, Mexico (7342 feet) are some examples of high altitude airports that have to construct very long runways to cater for more commercial planes. So, thinner air and higher elevation is not an advantage but rather a hindrance!

Many thanks to your input on simplifying the explanation about the difference between air speed and ground speed.

Also, thanks to your piece of trivia - that the testing of the earliest jet engines were conducted near Denver - on top of the world famous Pike's Peak (14,200 feet) to see if they would work at higher elevations - yes, jet engines do in fact work better in the thinner air. (Another story in another FAQ!)