Posts Tagged ‘Boeing’

When is 200 feet not 200 feet?

Monday, October 1st, 2012

This is probably something that you have not given a lot of thought, but think about this: When you fly a Cessna 172 at pattern altitude, say 1,000 feet, how high are you? What if you were flying in a Boeing 747? How high would you be if your altimeter in a 747 read 1,000 feet? Clearly, the entire airplane is not at that altitude, especially considering that the tail alone sits more than 60 feet off the ground when the airplane is parked.

If you are an instrument-rated pilot, or thinking about becoming one, one of the topics you will become familiar with is decision height or minimum descent altitude on an instrument approach. Considering that a standard ILS uses a published DH of 200 feet agl, which part of the airplane are they referring to? Does the pilot of one airplane have an advantage “over” another?

In transport category aircraft—that is, airliners—as well as most business aircraft, there is a radio altimeter that is essentially a radar for determining the height of the airplane over the ground. The crew needs to know exactly what is being referenced so that they can make an informed decision about executing a go-around.

The fact is that in larger airplanes, the radio altimeter computes the height above ground with reference to the wheels. This makes sense. Even on narrow-body airliners like the B-737 or the A-320, the crew might be sitting such that their heads—eyes—are 16 feet or so over the ground, which means that they are well over 200 feet agl at the lowest published altitude of the approach. For a 747, the pilot flying would be sitting even higher.

It only makes sense that all the required measurements are based on the height of the wheels. After all, it is the wheels that ultimately must cross the airport fence in order to assure a safe arrival of the plane. If you don’t believe me, just watch any one of the videos on YouTube of 747s crossing the beach and fence in St. Maarten (in fact, there is a great YouTube video that is filmed from the cockpit of a KLM 747 landing at St. Maarten).

As for who has the advantage? I’d say the 172 pilot does, for the simple reason that when a 172 pilot breaks out of a low cloud ceiling, the airplane does too. A 747 crew might well be still enveloped in a cloud while the wheels or even the lower row of passengers is in the clear. In reality, this will rarely matter if the airplane has a working auto-land system. But if it doesn’t, and the crew is forced to fly a standard Category I ILS, they might not see the runway, whereas the CRJ or even the 172 behind them does.

How about that…one-upping a Whale! Who’d a thunk it?—Chip Wright