GPS to the rescue!

The Flying Engineer seldom says never to getting to know everything about a flight. Some find it nosy, but very few actually realize what a great educational experience it actually is. Its through small activities such as this that an Aerospace Designer can truly “Get to Grips” with everyday aviation. Enjoy the read!

The "RDMI" display style page on the Garmin GPS

I just refuse to fly without my Garmin eTrex-H GPS. It’s a nice yellow handheld, with the front end sensitivity so great that it picks up GPS signals inside the metallic tube of an airliner with effortless ease. And it’s so accurate that I’d rather consider it as a personal backup navigation system in case an IRS-only equipped airplanes.

The "Integrated Instrument" like page on this basic Garmin handheld.

The display pages are phenomenal. Despite its monochromatic low resolution rendering of a user-defined database “map”, I land up feeling that I actually am in the second flight deck of the airliner. Curious passengers throw a mixed variety of looks, but I just don’t bother. This little device may very well save their lives one fine day.

So why the GPS? For one, I always want to know the extent to which the crew is following the route for the day: did they get a “direct-to”? Are they exercising a lateral offset to keep clear of the weather? What’s the ground speed, the track, and the bearing to the nearest VOR stations? What’s the vertical speed? I use this as my way to kill time, along with a nice appropriate Jeppesen chart.

Using the GPS

One high sensitivity Garmin GPS and a Jeppesen Chart are all you need! Ofcourse, a window seat as well!

The first thing I do is to initialize the GPS almost an hour before the flight. This prevents wasting valuable “locking” time when inside the airliner. I also open up the chart, and painstakingly punch in the latitude, longitude and identifier of the waypoint(s). Then I string the waypoints as part of a route. With that, I’m set to go.

I keep an eye on the taxi speed of the aircraft: it sure can let me know what type of a crew is flying the airplane: someone smooth or someone a bit impatient. And that surely reflects the captain’s mentality for the day, but not necessarily the first officer’s.

With my airport charts, I follow the taxi out as much as possible, keeping an eye out for the taxiway signboards. That gives me the situation awareness of where we are, what our sequence for take off could approximately be, and which runway we’ve got.

We got a "Direct To" when Enroute Surabaya-Kuala Lumpur on a Malaysian B737-400

On the ground, at not too great pressure altitudes, the ground speed and the indicated airspeed are almost the same, give or take approximately 10 knots for any wind. This allows me to get an idea of the rotation speed, Vr. By looking up the tables in the FCOM, I get a rough (note: very rough) idea of what our take off weight could have been.

Sometimes the SID beats me: leaving me behind the aircraft, unable to determine the SID for the day. This is when it gets a bit frustrating, but that soon passes. Before you know, the aircraft is headed to at least one of the waypoints on the “flightplan” that’s strung on the GPS, and things are back to normal.

Enroute Bangalore-Bangkok on board a Thai A300. Note the 33,000ft while we were actually at FL310. We were 7.6NM to Dawei (Burma)

I’ve always noticed about 2000ft difference between the pilot announced cruise altitude and the GPS indicated altitude. It’s kind of funny that it should happen very often, but it does. Following the aircraft is now simple, and I spend time with estimates, ground speed, and FIR boundaries.

I keep my ears pricked at about 200NM to destination onwards. As soon as the engine whines down, its time for descent. I check the V/S on my GPS to confirm this, and I note the Top of Descent point.

Note the approach parameters! Kuala Lumpur's VOR, "VKL" is my "reference" for DME to touchdown.

The fun again kicks in towards approach. Until the last few 50Nm or

GPS on the left, window view on the right: the Malaysian 734 on finals!

so, it’s really touch to gauge which runway you’d be landing on. By checking the aircraft track and referring the airport diagram of the destination airport, you can get your runway in use. the problem arises with parallel runways: you’ll have no idea which one is ready to burn your rubber. For that, you’ll need to either punch in the coordinates of the thresholds of both runways, or rely on the outside scenery after touchdown to determine your parallel runway.

The final approach is the best: the ground speeds are back to the familiar approach indicated airspeeds. The vertical speed is in close agreement with the usual 700+ feet per minute on the 3 degree glideslope. Since a VREF+margin may be used for touchdown, it is very hard to get the landing weight of the aircraft, as the “margin” is unknown. This point gets exciting, all the way to touchdown.

Bang. The wheels on the runway, the reversers coming out and the spoilers extended: the sights, sounds and forces are simply magical. Ladies and gentlemen, we have landed, on thankfully yet another uneventful flight that goes to show how safe air travel really is. And just in case….I’m always there, your NAV source 4 to your rescue!


3 thoughts on “GPS to the rescue!”

  1. Chow Kan Koo said:

    GPS altitude vs airplane flight level:

    Altimeters in airplanes are pressure instruments, converting outside air pressure into altitude above mean sea-level. They are calibrated to ISA condition, so they are only accurate when atmosphere condition is ISA +0. If air temperature is higher (as in summer) or lower (in winter), the indicated altitude in the cockpit would be higher or lower.

    As an example, flying at an indicated altitude of 31,000 feet, if outside air temperature is 10 degree Celcius higher than ISA condition (SAT is -37C instead of -47C), the actual altitude will be 32370 feet above mean sea-level, assuming that air temperature at various altitudes maintain this ISA+10 condition all the way from ground up (viz. air temperature is 25C at sea-level).

    GPS altitude is always accurate, as it is not affected by atmospheric condition.

    Chow Kan Koo

    • Did AF477 have a GPS running.? If yes, clogged pitot tubes , which indicate airspeed, should have triggered less panic in the cockpit and and crew should rely on GPS speed and altitude in such conditions

  2. Chow Kan Koo said:


    “If air temperature is higher (as in summer) or lower (in winter), the indicated altitude in the cockpit would be higher or lower” should read as:
    “If outside air temperature is higher (as in summer) or lower (in winter), actual altitude of the airplane would be higher or lower than indicated altitude in the cockpit”

    I apologize for the error.

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