Small Altitude Changes

[crowe]

Hello-

FlightAware has found its way onto my list of favorite sites. When tracking certain flights, I see that there are often tiny altitude changes that occur with varying frequency over the course of the flight. For example, a flight cruising at 36000 will momentarily descend to 35900 and then return to 36000.

I'm curious whether or not this is a result of a pilot testing for smoother air during a bumpy cruise. I guess it could just as easily be air traffic control determining it, but just curious.

Any thoughts?

[tb_neg]

100 feet isn't very much. It could be due to a downdraft which causes the plane to descend a bit, or perhaps they decrease speed but need to make minor adjustments to regain altitude. These aren't official answers, just my guess. I think it was discussed at one time in the flightaware forums.

[Smooth Sailing]

flightaware doesn't always report altitude accurately, but these readings could also be from altimeter changes

a flight reporting FL360 is actually higher or lower than 36,000 unless the altimeter is set to 29.92, because the airplane measures altitude by outside air pressure, not by bouncing RF signals to the ground

when air pressure's higher, the actual altitude the plane flies will go up, so a reported 36,000 ft is actually 36,100 or more, when it's lower than 29.92, the actual altitude will be lower than 36,000 ft

altimeter changes occur frequently during flight, and the pilot adjusts based on commands from the atc center, and typically when they enter a new sector the controller will read the altimeter setting as part of the greeting

when the pilot adjust the altimeter before adjusting altitude, plane will often be 100-200 feet higher or lower than assigned altitude because of the new altimeter reading, which would explain why you see movements like this on flightaware