Egpws flap inhibit

[buzzc152]

Hi all

We occasionally perform reduced flap landings that require us to inhibit the flap function of egpws (mode 4b I believe). Aside from silencing the 'too low, flap' call what actually is inhibited from gpws?
I've heard it suggested that using the flap inhibit switch removes some terrain floor protection.
Anyone more knowledgable than I care to comment ?
Many thanks.

[Skyjob]

What aircraft type are you talking about?

The only time I recall having to inhibit the Flap Inhibit Switch was when a new runway was constructed and the Terrain database was not yet amended, thus to allow a flight near the ground to be permitted approaching the *new* runway.

An EGPWS Terrain database allows you to fly near a runway (approach area) lower then it does outside this area. Thus inhibiting the Flap Inhibit in this case allows an approach without the alerts sounding when approaching short finals. There should be no other reason to use this switch in normal operations.

The logical step for operators who operate into arenas where such action is required for the above reason is to update their EGPWS database.

[wiggy]

What aircraft type are you talking about?

FWIW it's certainly a requirement on some of the Boeings if you're not using one of the standard landing Flap settings (e.g. the 777 if performing a Flap 20 landing for whatever reason).

[vilas]

Too low flap itself is a EGPWS function. Since you desire the reduced flap landing you inhibit that particular mode. That's it.

[Skyjob]

FWIW it's certainly a requirement on some of the Boeings if you're not using one of the standard landing Flap settings (e.g. the 777 if performing a Flap 20 landing for whatever reason).

My point exactly, non-standard!
No need for it in most cases...

[NZScion]

My type has two switches. An EGPWS flap override for use when landing with an abnormal flap configuration, and an EGPWS inhibit, for disabling callouts when landing at an aerodrome not in the terrain database.

[Gryphon]

Flap mode (“TOO LOW FLAPS” mode 4B) is inhibited to avoid nuisance warning in case of landing with flaps setting reduced. The normal landing flaps configurations in the software depend on the type.
To be precise, it is a basic GPWS function (modes 4A & 4B).

The enhanced function (EGPWS) provides terrain awareness by means of a specific algorithm, based on a worldwide terrain database, GPS, air data and radio altitude. If the airport where you'll land is not in the data base you should inhibit the enhanced function, again to avoid nuisance warning, but still you have the basic functions (modes 1-5)

[pilot1957]

Honeywell egpws guide advises that selecting flap inhibit also changes the mode 2 envelope from 2a to 2b. This occurs normally when selecting landing flap and puts mode 2 into a 'less sensitive mode' (my words).

[None]

What is the consequence of changing from Mode 2a envelop to the Mode 2b envelop? I don't have that information in our Aircraft Manual (Volume One).

This is what I found (without downloading the Honeywell manual):

•Mode 2B provides a desensitized alerting envelope to permit normal landing approach maneuvers close to terrain without unwanted alerts. Mode 2B is automatically selected with flaps in the landing configuration (landing flaps or flap over-ride selected) or when making an ILS approach with Glideslope and Localizer deviation less than 2 dots. It is also active during the first 60 seconds after takeoff.

By the way, the below is from the normal landing section in our Boeing manual:

• For FLAPS 15 landing,
• GROUND PROXIMITY FLAP INHIBIT switch - FLAP INHIBIT