View Full Version : GPWS Mode 6B

8th Nov 2009, 15:52
Studying Warning and Recording in Instruments for my ATPL's at the moment and I've come across a little odity.

According to the books, GPWS is only active between 50 and 2450 feet. Fine.

Radio Altimeter is active between 0 and 2500 feet. Also Fine.

GPWS gives height callouts and bank angle warnings between 50 and 2450 (mode 6B) yet I swear I've heard a "2500" ("twenty five hundred") callout before. Is the "2450 ft" rule just a minimum requirement or am I missing something that is probably horrendously obvious (or am I just plain wrong?!)



8th Nov 2009, 16:44
My experience is with Honeywell EGPWS Mk VI...

The only altitude callouts are 500, 200, 100, 50, 40, 30, 20, 10 and the "minimus minimums" call wherever the DH bug is set.

The radio altimeter does work up to 2500. But I have no idea if that is a legal requirement, a technical limitation or just a convention.
I know some Radio Altimeter boxes are labelled 10-5000ft. Never worked with those though. Sorry.
Someone here will know...

8th Nov 2009, 16:49
Greetings, the 2500 ft Radio Alt call exists we have it on the B777

8th Nov 2009, 16:57
:ok: Ok, I believe you. My plane is a little smaller than your triple7... :}

So, it seems size DOES matter after all :{

Spooky 2
8th Nov 2009, 17:11
Typically GPWS/RA callouts can be pinned or "strapped" to the customers specs. So the 2500 RA, may or may not actually be an audible depending on what the operator has selected for their specific operation. Same goes for items like "approaching minimums" along with the various alt. callouts close to the ground. i.e., 100, 50, 40, 30, 20, 10 and so on.

8th Nov 2009, 17:23
Ah the way it's written in the book makes sense now! So "Bank Angle" is the only mandatory callout in 6B, but the rad alt callouts are essentially configurable/optional.

Thanks for the help!! :ok:


9th Nov 2009, 15:22
It's tough to write a question to cover all types and vintages of aircraft. Audio source can be the TAWS, the radio altimeter, or something like the OWL card in the 757.

Earlier Arinc conforming radio altimeters would track from about 2800 feet on down, while the display would be limited to 2500 feet. Newer (767 & subsequent designs) Arinc 700 radio altimeters track from about 5,000 feet, but again, the display is limited to 2,500 feet, as the original design intent of the radio altimeter is as a landing aid from about 200 feet on down.

All other functions, such as Autothrottle Retard, are afterthoughts, and often not well thought out, as maybe shown by the Turkish 737 at EHAM.


10th Nov 2009, 16:36
Typical indication range of Radio Altimeter (or Radar Altimeter) is up to 2500 ft but the system works fine at higher levels too - e.g. if you look at the preliminary report about Turkish B737 crash at Schipol they even mention radio altitude of 8191 feet there.

10th Nov 2009, 17:37
And indeed its the same on the Airbus, the Rad Alt height is displayed on the PFD <2500ft, but the system kicks in well before that.

Don't pin me down on the exact height, but I believe it is somewhere in the order of 5000ft.

One of the things you can notice this by is the fact that if you press the approach pushbutton to arm the Loc / GS above 5000ft, you will only get CAT 1 displayed, when descending through 5000ft the rad alt kicks in and you get CAT 3 Dual, if you're fully serviceable that is.

10th Nov 2009, 19:32
All advisory calls are operator specified. My last airline had no height calls on the 737 - it was all done by pilots, and I still find the height read outs a little annoying when landing.

The 2500 foot call is specified, or sometimes manually applied as an SOP, as it is below this height that mode one "Sink Rate" calls (at rates of descent of roughly twice the radio height) become active. It's a trigger to be aware of your sink, and control it so that a GPWS call isn't made.

I have seen rad alt displays (old ones, with a needle and dial) which ran above 10,000 feet. Why? They were used for Pressure Pattern Flying - a navigation technique in which accurate heights above sea level (on over-water legs) are compared to changes in pressure (from the pressure altimeter) and the difference can be used to calculate your drift angle. The calculation markings for this can still be seen on Jeppesen CR series flight computers.