GPS to the Rescue!

The author said he would not fly without his Garmin eTrex H, a small yellow handheld GPS. It is sensitive enough to pick up satellite signals inside an airliner's metal tube, and accurate enough to act as a personal backup navigation system on an aircraft that has only inertial navigation. The first idea was that a curious passenger can learn a great deal this way, which suits an aerospace designer who wants to get to grips with everyday flying.

Why carry a GPS

The display pages show a simple map from a user made database, with pages that look like an instrument panel. The author said it makes him feel as if he has a second flight deck. He uses it to see how closely the crew follows the planned route. Did they get a direct to? Are they offsetting to avoid weather? He also reads ground speed, track, bearing to nearby VOR stations and vertical speed. With a Jeppesen chart, it is his way of passing the time.

Setting up

He switches the GPS on about an hour before the flight so it has locked on before boarding. He then types in the latitude, longitude and identifier of each waypoint from the chart and strings them into a route.

On the ground and in the climb

• Taxi speed hints at the crew. A smooth taxi or an impatient one tells something about the captain's mood that day, though not necessarily the first officer's.
• With airport charts, he follows the taxi route by reading signboards. This tells him where the aircraft is, roughly what the takeoff order is, and which runway is in use.
• Near sea level, ground speed and indicated airspeed are within about 10 knots of each other. That gives a rough idea of the rotation speed, Vr. Looking this up in the FCOM tables gives a very rough idea of the takeoff weight.
• Sometimes a departure procedure is too quick to work out. The frustration passes once the aircraft heads for one of the waypoints in the route.

Cruise

The GPS altitude often differed from the announced cruise level by about 2,000 feet. One photo taken on a Bangalore to Bangkok flight on a Thai A300 showed 33,000 feet on the GPS while the aircraft was at FL310, 7.6 nautical miles from Dawei in Burma. In cruise, the author tracked estimates, ground speed and FIR boundaries. Another photo was taken on a Malaysian Boeing 737-400 from Surabaya to Kuala Lumpur, which received a direct to.

From about 200 nautical miles from the destination he listens for the engines to spool down, then checks the vertical speed on the GPS to confirm the descent and notes the top of descent point.

Approach and landing

In the last 50 nautical miles or so, it is hard to tell which runway will be used. The track and the airport diagram help. Parallel runways are a problem. One option is to enter the thresholds of both runways, or to wait until touchdown and look outside. The author used the Kuala Lumpur VOR, VKL, as the reference for DME to touchdown.

On final approach, ground speeds come back to the familiar approach airspeeds, and the vertical speed fits the usual 700 or more feet per minute on a 3 degree glideslope. Landing weight is hard to guess, since crews may add a margin to the reference speed and the margin is unknown. The author ended on a lighter note: after the touchdown, the reversers and spoilers, the GPS remains the "NAV source 4", ready in case it is needed.

Reader comments

A reader explained that cockpit altimeters are pressure instruments calibrated to the standard atmosphere. When the air is warmer or colder than standard, the true altitude differs from the indicated one. For example, at an indicated 31,000 feet with air 10 degrees Celsius warmer than standard, the true altitude would be about 32,370 feet. GPS altitude is not affected by this. A later reader asked whether Air France 477 had a GPS running, and whether crews should use GPS speed and altitude if the pitot tubes are blocked.