GPWS Minimums with MDA

[FLY744F]

Hello

How would i get the Minimums sound to play when using MDA, it plays when using DH but need it for my BA setup iam using plus 100 for the 50 above but my  Minimums wont play.

George

[Mariano]

George,

In the 767, the GPWS calls minimums only when it reaches the DH set on the EFIS Control Panel. Since the 767 does not have an altitude tape, the MDA bug is purely mechanical (just like the standby ASI bugs in the 744.)

I may be wrong, but in the 744, I don't believe the GPWS is supposed to announce minimums when reaching MDA either (even if the bug is "inside" the EFIS altitude tape.) DH (if the airplane sticks to the glide slope) is reached at a specific point in (lateral) space for that particular procedure every single time. So, it makes sense that a GPWS minimums call will be quite accurate.

An airplane doing a non-precision approach may reach MDA at many different points, depending on speed, descent rate, glide path angle, etc. and whether the airplane flew a "drive-and-dive" approach or a Boeing-recommended Constant Descent Approach Procedure (CDAP.)

To simplify, since DH is always reached at the same spot for all types of airplanes doing that approach (and MDA is not.) MDA is not announced by the GPWS, only by the PM referencing the barometric altimeter.

I have never flown an airplane in which the GPWS calls out the MDA setting. You could set your DH to the MDA height equivalent (shown in parenthesis next to the MDA) but I have yet to see anyone do that. I think it is there for reference. I believe that a non-precision approach should be treated as such; DH is a precise concept, MDA is not.

I might be wrong so any corrections are more than welcome.

Hope this helps,

Mariano

[FLY744F]

Hi

Thank you for your reply.  In a few ba 744 landing videos on YouTube the crew are using baro/mda and they get gpws callouts of  50 above callout then decide at mda is this added just to the ba fleet?

Fly744f

[Mariano]

George,

Thanks for the correction and information regarding BA's 744s. I guess the it must be an option selected by them. I have heard the "50 ABOVE" and "DECIDE" callouts on BA videos. I just always thought they related to DH.

Thanks,

Mariano.

[John H Watson]

The 744 Maintenance Manual mentions that this is an option (DH and/or MDA).

In older manuals for BA, the FIFTY ABOVE and DECIDE callouts are restricted to "YGB-ZZZ" (I assume this means G-BYGB to G-BYGG)

Rgds

[Jeroen Hoppenbrouwers]

Calling "decide" at MDA feels wrong. You stay at MDA until you see the runway environment -- or not. There is no single "decision point", unlike at DH, where you have a definite moment to say LAND or GO AROUND.


Jeroen

[Hardy Heinlin]

From memory (correct me if I'm wrong):

Optional call-outs for MDA (minimum descend altitude at any DME):
- "Plus hundred"

Optional call-outs for passing DH (descision point on glideslope):
- "Decide"
- "Minimum"
- "Minimums"

Optional call-outs for approaching DH (descision point on glideslope):
- "Fifty above"
- "Approaching minimum"
- "Approaching minimums"

I can't remember if the latter is a GPWS sound or a human PNF. I have the PS1 sound in my head, but Mel's voice sometimes sounds like a GPWS voice and I might confuse something here :-)


Cheers,

|-|ardy (not yet working in this area in PSX)

[Hardy Heinlin]

Did BA disable the "Plus hundred" GPWS call because they wanted +50 instead of +100 and because there is no +50 option in the GPWS box the BA crew must call it personally?