E-GPWS modes 1-4 with GPS deferred.

[West Coast]

On your aircraft with GPS deferred are there any references in the MEL to E-GPWS accuracy in modes 1-4 for terrain avoidance?

In plain language, minus GPS, are the remaining nav inputs to the FMS you fly sufficient to provide position fixing to still be accurate to the same degree

Broken down even more, minus an obvious sign like a map shift, are you sure that E-GPWS terrain warning received is truly on your path of perhaps offset due to accuracy?

I know I will react as if it was, but curious if the accuracy minus GPS is addressed in your MEL.

[I-2021]

Hi West Coast,

On your aircraft with GPS deferred are there any references in the MEL to E-GPWS accuracy in modes 1-4 for terrain avoidance?

Very interesting point. Having a look at the MEL for our A320/330 equipped with the latest TAWS architecture the answer is no, although the main position reference for terrain display is the GPS position.

In plain language, minus GPS, are the remaining nav inputs to the FMS you fly sufficient to provide position fixing to still be accurate to the same degree

They are but...

Broken down even more, minus an obvious sign like a map shift, are you sure that E-GPWS terrain warning received is truly on your path of perhaps offset due to accuracy?

Here's the point. You may be subject to spurious or false alerts as the main source of informations in the absence of a valid GPS input would be the actual FM position, which in case of a map shift (or erroneous FM position) could lead to false alerts.

I know I will react as if it was, but curious if the accuracy minus GPS is addressed in your MEL.

As far as I can see (hopefully somebody else will be more specific) there is no reference to that in the MEL.

[safetypee]

May depend on the EGPWS equipment manufacturer (Airbus offers a choice); also if the EGPWS has a GPS receiver embedded in the box vice an external input (Honeywell option).
Failure of the embedded GPS would presumably give an EGPWS fault.
Dispatch without an external GPS might depend on the accuracy of the remaining navigation source to be used for the dispatch and how EGPWS uses this. Furthermore if that nav source degrades – e.g. no DME cover, then system status/alerts might be given which could affect flight operations.
Read the MEL and dispatch deviation guide (DDG) carefully and ensure that you understand any implications or restrictions.

[cosmo kramer]

GPWS mode 1-4 are radio altimeter based alert and does not rely on GPS.

At least for 737NG, don't know if other aircraft manufactures differ, but I doubt it. :)

TAWS (Look–Ahead Terrain Alerting) relies on GPS and may be inoperative for 10 days if GPWS warnings are operative.

[safetypee]

‘GPWS mode 1-4 are radio altimeter based alert and does not rely on GPS’.
Correct, but the Enhanced components of EGPWS may still be available based on alternative position data.

Extract from Airbus operational information tetex; SE 999.0015/04/VHR, 5 Feb 2004
This outlines their policy in only certificating an external GPS, preferably as part of the MMR.

“The TAWS is an improved system over the existing GPWS (ground proximity warning system). TAWS improves on existing systems by providing the flight crew automatic advanced aural and visual display of impending terrain, much earlier warning, forward looking capability, and operability in landing configuration. These improvements provide more time for the flight crew to make smoother and gradual corrective action.
To achieve the new terrain functions, the TAWS computer uses the aircraft position to locate the aircraft on its internally loaded terrain database. This aircraft position is currently provided by the flight management system (FMS) that computes it:
1) from the ADIRUS and GPS position source for aircraft equipped with GPS sensor unit (GPSSU) or multimode receiver (MMR), when GPS primary is available, or
2) from the ADIRUS and radio position update, for aircraft not equipped with GPS, when ground radio navaids are available, or
3) from the ADIRUS only, for aircraft not equipped with GPS receiver when radio position update is not available.”

Presumably paras 1) and 2) would be part of the MEL / DDG for aircraft with GPS; if the GPS was u/s then EGPWS functions should s be available.

[West Coast]

2021,
My MEL doesn't note it either, thanks

Safetypee
Interesting point about an embedded GPS. Never thought of that. It's a Honeywell box but all I have access to is a generic user guide that doesn't address differing models.

Cosmo
As noted, GPS does play into the equation. Especially for mode 4 which is where my primary concern is.

[cosmo kramer]

West Coast
Cosmo
As noted, GPS does play into the equation. Especially for mode 4 which is where my primary concern is.

No it doesn't:


Mode 4 - Radio Altitude Based Alerts:
TOO LOW, TERRAIN
Unsafe terrain clearance at high airspeed with either landing gear not down or flaps not in landing position. Follows DON’T SINK if another descent is initiated after initial alert, before climbing to the altitude where the initial descent began.

TOO LOW, FLAPS
Unsafe terrain clearance at low airspeed with flaps not in a normal landing position.

TOO LOW, GEAR
Unsafe terrain clearance at low airspeed with landing gear not down.


TAWS - Look Ahead Terrain Alerts (GPS):
TOO LOW, TERRAIN
Descent below unsafe radio altitude while too far from any airport in the terrain database.

CAUTION TERRAIN
40 to 60 seconds from projected impact with terrain shown solid amber on the navigation display (in expanded MAP, center MAP, expanded VOR, or expanded APP modes only).

TERRAIN TERRAIN PULL UP
20 to 30 seconds from projected impact with terrain shown solid red on the navigation display (in expanded MAP, center MAP, expanded VOR, or expanded APP modes only).


The aural alert "TOO LOW, TERRAIN" is used by both systems, but still it they are two independent systems. If TAWS doesn't work (for any reason), GPWS mode 4 alerts are still available as they are only radio altimeter based (+ combinations of barometric altitude, airspeed, and airplane configuration).

All the above is pretty much copy paste from the OMB. :8

[safetypee]

Depending on the level of technical detail being considered perhaps both views of GPS’s use in Mode 4 have merit.
GPS appears to be used in Mode 4 as in the text below. If GPS is not available we might assume that Mode 4 reverts to the basic system – it works as the old GPWS, but EGPWS may still be available via an alternate nav source.
This reversion probably does not warrant any MEL comment, but there should be some reference to the alternate nav source for EGPWS if the nav accuracy is less than GPS.

Extract from Honeywell Pilot’s Guide MkV & VII,
‘With version -210-210 and later models, Mode 4 airspeed expansion is disabled (upper limit held at lowest airspeed limit) when the Terrain Alerting and Display (TAD) function is enabled and available. This is due to the enhanced alerting capability provided with TAD, resulting from high integrity GPS Altitude and Geometric Altitude data. This change to the Mode 4 envelopes reduces the potential for nuisance alerts when the aircraft is not in the landing configuration.’

[Intruder]

744 and 748: no cross references in the GPWS or GPS items in the DDG.

[West Coast]

Cosmo

I'll be the first to admit I'm not one for what goes on beyond the front of the box in most cases, I just simply comply with E-GPWS protocols set forth in my SOPS. I read the same excerpt that safetypee has kindly posted as I fly with a Honeywell product as well. Other areas of the manual available on my companies website also allude to GPS being a component used in E-GPWS.
Am I understanding you correctly to say GPS has no input to E-GPWS? Specifically mode 4.
Under GPWS I would agree regarding the RADALT but I've been taught (perhaps improperly, perhaps oversimplified) that E-GPWS is based in part off a data base that compares position against a data base and ADC input and warns you of the terrain ahead, sometimes a fair distance ahead. This being something a RADALT has no way of doing accurately.

Please school me up if I'm wrong.

[I-2021]

Some useful info (that you are probably already aware off) here (www.cockpitseeker.com/wp-content/uploads/A320/pdf/data/gettingToGripsSurveillanceIssue1.pdf) from page 118 to 129.

[cosmo kramer]

West Coast,

I think it is safe to consider them two independent systems, even if they are isolated through electronic logics inside the same box. The reason is that the 737 MEL has the following separate items:

Ground Proximity Warning System (TAWS)
1) Modes 1 thru 4
"May be inoperative for a maximum of 6 flights or 25 flight hours or 2 calendar days, whichever occurs first."

4) Terrain Awareness & Warning System (TAWS)
May be inoperative for a maximum of 10 calendar days provided the GPWS functions are operative.

and

5) Terrain System – Forward Looking Terrain Avoidance (FLTA) and Premature Descent Alert (PDA) Function
May be inoperative provided alternate procedures are established and used.

Glideslope, windsheer, and advisory callouts have separate items too. Probably as the box is able to isolate each mode if a sensor input is missing, instead of rendering the entire GPWS inoperable.

I don't know what you think about the reliability of the following source, but as you can see, they list terrain proximity as a "not numbered mode" (scroll 2/3 down):
SKYbrary - Terrain Avoidance and Warning System (TAWS) (http://www.skybrary.aero/index.php/Terrain_Avoidance_and_Warning_System_(TAWS))

On another note, some years ago a friend on mine landed on a newly built runway that wasn't in the TAWS database. Since the new runway was at a considerable distance from the old one, he got "TOO LOW, TERRAIN" warning during approach. Clearly a TAWS function (descent below unsafe radio altitude while too far from any airport in the terrain database), because no mode 1-4 would have been triggered during a normal stabile approach. So this is an example of how it works in practice.