Why do Airplanes Fly at High Altitudes?
February 23, 2010 at 1:06 am Chad Upton 1 comment
A few years ago, I was on a short, low altitude flight on a cold December evening in the Northeast. Because it was a short flight to a tiny airport, the aircraft was a small turboprop plane.
The flight was bumpy from the start. Somewhere in the middle, the flight crew was providing beverage service when the captain turned on the fasten seat belt sign. He instructed the crew to return to their seats and buckle up; before they made it to their seats, we hit a low pressure air pocket and the plane took a sudden drop.
It was just like a roller coaster making its towering first drop. We were in free fall for what felt like a really long time, but was probably only a couple of seconds. A few people flew out of their seats and hit their heads on the overhead bins, the crew was in the isle and did the same. Drinks, books, purses and other personal articles were thrown around the cabin, making a huge mess.
Luckily, nobody was seriously hurt.
That incident showed me exactly how dangerous bad weather and low altitude flying can be. It also demonstrated exactly why you should wear your seat belt when that light is on — it’s not a suggestion, it’s an important safety measure.
While we might not think about the weather and altitude much, they’re front of mind for the pilots. In fact, they review full weather reports that cover multiple altitudes for the entire route before takeoff.
The weather reports help them decide which altitude (or combination of altitudes) they should fly at. There are generally fewer clouds and less turbulence at high altitudes, but there are still some of both. Some clouds (towering cumulus and cumulonimbus) can extend as high as 60,000 feet — well above typical air travel.
At least the clouds are easy to see, some other phenomena cannot been seen by the pilots so they rely on radar and satellite reports to avoid those when possible. Other environmental issues cannot be seen, including Clear Air Turbulence (CAT).
The engines are also optimized for high altitudes, so fuel efficiency is another benefit of climbing high. For many engines, they can run just above idle at around 40,000 feet. That’s because the air is thinner, which means the engines have less resistance and the plane gets more lift with less thrust.
Because the air is thinner, there is also less ice accumulation at high altitudes. Ice buildup is dangerous and caused a tragic plane crash outside of Buffalo, NY last year (Colgan Flight 3407). Reduced ice buildup is an added benefit of high flying.
The high altitudes are also great for separation of flights to avoid collisions, you can read more about that in my related post, “Why Airplanes Don’t Always Fly in Straight Lines to Their Destination.”
There are also some downsides to flying high. As I said, the atmosphere is different, which means people cannot breath normally at this altitude. Aircraft that travel above 10,000 feet should provide a pressurized cabin or a supplemental air source for the pilots and passengers (or both).
As you can see, there are many reasons why planes fly at high altitudes, and now you’ll understand why the pilot thinks it’s important to tell you about the weather and altitude of the flight.
Broken Secrets
Written By: Chad Upton
Related Post: Why Airlines Dim Interior Lights Before Night Landings
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Photo By: Olastuen (creative commons)
Sources: FAA (PDF), Jet Aircraft, Dept of Energy, Flight 3407, Fear of Flying
Entry filed under: Demystified, Travel. Tags: airplane, altitude, bump, efficiency, flight, jet, plane, Travel, turbulence.
Broken Secrets 
1.
Janec72 | April 15, 2010 at 12:34 am
Chad, kudos for writing a fairly concise description of the effects and benefits of high-altitude cruise on airplanes.
However, there is no such beast as a “low pressure air pocket”, nor was the turbulence you experienced the fault of the relatively small size of the aircraft.
What your flight experienced was Windshear — an abrupt change in wind velocity and/or direction. Any aircraft in flight has momentum, and when the airmass through which it is flying is “squirrelly” (due to thunderstorms or cold fronts, for example), the aircraft can abruptly gain or lose airspeed and/or altitude. The aircraft keeps going (momentum), while the air shifts around it (windshear), and the result is “turbulence”.
We humans are particularly sensitive to the sensation of dropping (more so than to the sensation of rising, for some reason), and indeed the sudden loss of altitude can feel like falling into an empty “pocket” devoid of air. In our Earth’s atmosphere, however, an empty air pocket is not physically possible.
There is no pocket, no emptiness — only a very temporary reduction in lift. The momentum of the aircraft will carry through and the lift over the wings will be fully restored within seconds. Aircraft are designed to be inherently stable and self-recovering from turbulence. We might have a bumpy ride, but the aircraft will stay right-side-up and aloft.