View Full Version : Back Seat Driver?

12th Jun 2003, 12:41
Saw this message on a GPS newsgroup. Watch the accuracy of your P.A. announcements ladies and gentlemen!


On a Southwest Airlines flight from Nashville to Houston yesterday, I was
using my GPS and had a very good satellite lock with the altitude showing
39000 feet.. The captain announced that we were going to climb to 41000
feet to try to get above the choppy weather. We climbed to 43000 feet where
we leveled off and flew for about 15 minutes. Then, the captain announced
that he was sorry, but it appeared impossible to get high enough to smooth
out the ride, that we were at 41000 feet and that was the maximum height for
the 737 we were in. My little yellow etrex still showed a solid 43000 feet.
We continued at 43000 feet for about 10 more minutes until we abruptly
descended to 41000 feet where we continued until we began our descent into

I didn't get any particularly warm fuzzies from the knowledge that the crew
on the flight deck flew for almost one half hour thinking that we were 2000
feet lower than we were. I don't know what the vertical separation between
planes is supposed to be. Were we in danger because of this error?


12th Jun 2003, 12:58
The GPS was not showing the aircraft's altitude. It was showing the receiver's height above the WGS84 ellipsoid - a theoretical model of the earth. The pilot may have announced the aircraft's altitude as 41000ft, but it was not. It would have been Flight Level 410, which is 41,000ft above a standard pressure level of 1013.2 HPa - a nicety that most passengers are unaware of, and it's not really important (to the passengers) anyway.

Sorry, you can inform the author that he and his receiver aren't that clever!

12th Jun 2003, 16:05
Fishing Journo perhaps???? :confused:

12th Jun 2003, 16:19
Got to Malaga not so long ago and a PAX asked to speak to us, Nice chap said that he had used a GPS to follow our progress and that he was surprised when we said we were cruising at 35,000ft as his GPS said we were cruising at 36,000 plus ft, Captain explained the same as above posting...

12th Jun 2003, 19:35
...why was he operating a radio reciever in the cabin?

Anti Skid On
12th Jun 2003, 19:40
Mine doesn't work in the car or in the house, so how the hell did they get theirs to work inside an airframe?

Sick Squid
12th Jun 2003, 20:51
... a little knowledge is a dangerous thing. Moving to Tech Log....

Golden Rivet
12th Jun 2003, 23:08
not sure the WGS84 model would be that far out - as flight levels are based a 1013.25 mb standard it is more than likely that the sea level pressure was not at 1013, resulting in the difference.


13th Jun 2003, 14:38
GPS is inaccurate enough vertically to generate enough error, plus the FL/Alt conversion on top of that, plus a teensy bit of altimeter error, plus probably a tonne of receiver error in their little e-trex :)

Anthony Carn
13th Jun 2003, 15:14
vertalop would appear to be in need of a CRM course (assuming, however, that CRM courses did any good in the first place) !

If he thought that there was a piloting error, then why did he not raise this with the cabin crew, who would have relayed it to the flight deck ? :confused:

Echoes of "I thought he knew which engine it was - duhh !" :rolleyes:

In general, if you're a passenger and something does'nt seem to be as it should, then tell a member of the Cabin Crew (the one in overall charge is probably best). IMHO :ok: