# Bored At 40,000 Feet

I was flying home yesterday and I was bored. I reached for my tablet and started playing around with a function I seldom use while home, GPS. With the Global Positioning System my tablet figures out where I am by measuring how long signals from a constellation of Earth orbiting satellites take to reach it.

This is mind boggling math done instantaneously. I’ll spare you the details. Just say it’s magic! That works for me.

Surprisingly, after a few minutes the tablet locked on and started reporting the plane’s position and speed.

We were flying over Imperial, Nebraska. Our track was 257°, a little south of west. The ground speed was 484 mph. Our altitude was 41,110 feet.

Uh oh. That couldn’t be right. The pilot said we were assigned 40,000 feet. Flying 1,110 feet higher could cause problems.

I checked the FAA’s records this morning. They agree with the pilot, 40,000 feet. Could my GPS have been that wrong?

Truth is we were probably closer to 41,110 feet than 40,000 feet. But it doesn’t matter because the altitude the pilot gives is hardly ever correct!

Here’s where weather enters in.

Years ago before fancy electronics and satellites, pilots needed a way to know how high they were flying. The simple answer was to measure atmospheric pressure. We already had an instrument for that, a barometer.

As you climb higher the atmosphere become less dense. The barometric pressure gets lower. Calibrate the barometer to measure this pressure fall in feet and you’ve got an altimeter. There’s one (or more) on every plane.

Of course it’s never that simple. The barometric pressure over any spot on Earth is constantly changing. The barometric pressure between any two points also varies. If pilots set their altimeters at different airports they’d constantly be at different altitudes. The skies above us would be chaotic!

A plan was devised. Once a plane is airborne and above 18,000 feet the barometer reading is reset to 29.92″, no matter what the ‘real’ pressure is. Once my plane was set to this ‘standard,’ its 40,000 foot reading matched all the other planes claiming 40,000 feet.

Disaster averted, even though none of them were actually at 40,000 feet.

This system works beautifully for high altitude jets in flight–not so well near the ground. Planes taking off and landing reset their altimeters to the actual pressure at the airport so they know exactly where the runway is.

This system has been in use for decades and continues today in spite of technological advances, because it works!

### One Response to “Bored At 40,000 Feet”

1. Neil S says:

We actually set the altimeter when at the airport to local station pressure so that we know what the elevation of the airport is. The reference elevation from where the station pressure is measured, can actually differ considerably from the elevation of the various runways at the airport. Case in point, DEN has several runways all with varying elevations due to the rolling terrain that the airport is built on. For precise measurement of elevation near the runway, we use ‘radio altimeters’ which measure precisely the height of the aircraft above the runway.