737 NG - Position trend vector vs. GPWS Switch

[pharmair]

During a visual apch into a very tight valley somewhere, two of my colleagues turned off the GPWS Terr Inhibit toggle switch on the FO side in order to avoid too many inputs. Apparently, this inhibited the Position Trend Vector on the ND as well - a feature which is nice to have in tight spots.
I can´t find any reference to this in any manuals - can anybody confirm the relationship between these two, and explain the background for it?

[Kirks gusset]

I cannot comment on the original question as it is beyond the scope of my knowledge, however, I would be interested in how these Inhibiting procedures are worded, simply, because I have never seen an Ops Manual which allows it.

We all know there is an ICAO and EU OPs regulation stating it must be fitted : 1.665 and it contains the following guidance:

6.3.5
Instructions on inhibition must include a statement that no person may deactivate the GPWS except in accordance with the procedures stated in the Operations Manual. It is essential that this is made clear as there have been accidents in which GPWS had been inhibited in order to silence what were genuine warnings.

Could you cut and paste the wording from your mates manuals please.

KG

[Piltdown Man]

The whole point of having EGPWS is to prevent aircraft from having too high closure rates with terrain. If the system is U/S or there is a tested, trained and approved procedure to turn it off then fine, but to do so because the "system complains too much" is bordering criminal behaviour and thus indefensible by unions and accident investigators.

These sorts of heros should mend their ways.

PM

[SlowAndSilly]

Sorry for not answering about the trend-bit, but in my "former" airline we were in a few places instructed to degrade/inhibit the "E"-part of the EGPWS to avoid nuisance-alerts/cautions.

These instructions were part of the OM-C.

[Denti]

That can happen, however as far as i know it should only be a temporary measure until the database is updated to prevent nuisance warning during normal operation. Usually that does not take longer than a week or two, of course only if the airline shares its complaints with the database provider and pays for a new one.

[ad-astra]

OK High Horses are now stabled and perhaps we can NOW answer the question.

[pharmair]

Thanks to the flight instructors out there for their advice on how to operate a 737 safely. The pilots mentioned were not born yesterday, nor did they start flying yesterday either....

The airport in question is day only, VFR only. Due to it´s high elevation and relatively short runway, it is very important to have a stabilized approach when operating a 737 there. On short final there is a hill that interferes with a straight in apch to one of the runways. Most smaller aircraft fly around the hill; my mates decided to overfly it with a margin that, although safe, would have the GPWS blaring through the last few seconds before touchdown. They decided to turn it off for a few seconds.:eek:

Now, can somebody please answer the TECHNICAL question that I asked...?

[c100driver]

The EGPWS inhibit switch just inhibits the E part of the GPWS system, the remaining modes of the GPWS are still active.

I dont fly the NG but would take a guess that the inhibit removes the terrain data base from the ND which may be why the vert position trend vector is removed?

Some airports have significant close in terrain that may be difficult to map, or the nav solution not accurate enough for the terrain database that can cause false warnings. I operate to two such airports and we are authorised in VMC on finals to select overide.