EGPWS altitude reference

[discus2]

Hi,
What is the EGPWS using for altitude reference ?
The RA ?
1013 ?
How are the too low flaps/ gear alerts computed?
Does the actual QNH setting on the altimeter have an influence on such alerts?

Many thanks!!

[ATCast]

EGPWS used primarily GPS height above mean sea level for vertical reference, usually augmented by a barometric source for short term variations.
Altimeter setting has no effect on EGPWS.

[Denti]

That depends on installation I guess. Our 737s are not certified for QFE operation since the EGPWS is based on QNH barometric input.

[BOAC]

Operation - EGPWS - Honeywell Aerospace

Pilot guides

[safetypee]

The primary reference for terrain and obstacle awareness and avoidance is Radio Altitude.
The gear / flaps alerts come from comparing the selected position against RA – gear normally < 500ft, flaps < 200ft – flaps can be overridden by selection.
Baro / GPS inputs as per model and installation; some restrict QFE ops others provided baro comparator from GPS.

[Skyjob]

safetypee, your references to RA are very valid but have no relation to the EGPWS input source for terrain awareness.

BOAC, thanks for those references.

EGPWS takes its data from GPS position, in short, thus independent of barometric settings and not prone to a QNH blunder error. VSD (if installed) uses EGPWS inputs to display its terrain data as well, again providing a safeguard for blunder error (as runway symbol will either float in air or be buried under ground reference)

As mentioned in the Honeywell documentation:

Units are avail- able with an internal GPS receiver for required GPS data when another GPS source is not available.

[Denti]

Sadly the manuals we have are not all that good on systems, but interestingly enough that is the quote from our limitations section:

QFE Operation
QFE operation is prohibited on the main aircraft altitude system due to the fact that EGPWS data are based on QNH.

Only thing about barometric inputs in the technical documentation is this single sentence about the look ahead terrain feature:

Look-ahead terrain alerts are based on the airplane’s position, barometric altitude, vertical flight path, and ground speed.

All in all it seems barometric inputs are used directly by parts of the EGPWS we have installed, other installations might differ of course. On our old 733s which did not have GPS input to the FMC the EGPWS had their own internal GPS and were indeed independent of barometric inputs.

[Skyjob]

Denti, are you sure you have on those aircraft EGPWS or GPWS installed?

Reference: "EGPWS has all the capabilities of GPWS and augments it by using a Global Positioning System, or GPS, to provide very accurate information on the exact location of the aircraft. This is then coupled with an extensive terrain database; basically, a map that details how the ground changes in the area."

[catiamonkey]

EGPWS has all the capabilities of GPWS and augments it by using a Global Positioning System, or GPS

Not necessarily. As Denti states, older types have a EGPWS box with an internal GPS. You have an interesting situation though types like the A320. These feed the EGPWS box with IRU data. If the airplane has GPS, then this works as you'd expect. However, if the airplane does not have GPS (old A320s), then EGPWS is dependent on proper FMS/IRS updating of position.

I remember an incident in Africa where a malfunctioning VOR almost caused a non-GPS A320 to fly into a hill, and the EGPWS function didn't say a thing.

[underfire]

In regards to other posts that mention built in terrain models, this is true, and really this is the E in EGPWS. It uses a Geometric Altitude, based on GPS altitude, coupled with a terrain model lookup feature, that computes a pseudo-barometric altitude. This is designed to reduce or eliminate altitude errors resulting from temperature extremes, non-standard pressure altitude conditions, and altimeter setting mistakes.
The method for the 'lookup' feature varies. Some systems use a grid lookup to the model (typical), while others use the lat/long location.
As far as how 'extensive' the terrain model is, I have seen little other than the WGS84 ellipsoid, with added/subtracted known issues...the FMS database is relatively limited in data size, (as in very little storage capability)

Some systems correlate with procedure leg. As an example, if you are on the intermediate leg, and have less that a 500' ROC, it will alert...
Other systems correlate with distance from the runway, for example tapered from threshold to a 400' ROC from 5nm to 12nm, tapering to 7000' ROC at 12nm. (from this, one can see that if you have a FAF at 5nm, one system will have a ROC at 500, while the other 400, thus different alert potential)

The look ahead includes the precipitous terrain feature, which uses the radalt input. This is why when you have terrain that suddenly rises, especially on final, the system extrapolates that as a continuous rise and will alert.

EDIT: catia.. I am not seeing how a non-gps A320 could have EGWPS...Typical GPS systems are connected to the IRU, not to the FMS. The incident you mention, with lack of response from the GPWS system, it seems the forward radar look ahead would have seen the obstacle...

[BOAC]

Denti tells you!

On our old 733s which did not have GPS input to the FMC the EGPWS had their own internal GPS and were indeed independent of barometric inputs.

. Flown a few of those.

[safetypee]

EGPWS has advanced features of a terrain database enabling a look-ahead function, a terrain-floor feature, and airfield/runway recognition; there are also many software mods to the original warning system which maintains the very high reliability of the alerting system.
The first EGPWS's used the best available high-accuracy navigation input; those aircraft with DME/DME or IRS (or GPS failure) could suffer a map slip – the conditions of susceptibility and/or geographic location were alerted in the AFMs; this also included issues of QFE.
In more recent aircraft, EGPWS nav input can be from a range of external sensors – GPS being the most accurate, but the EGPWS also has an optional (the better overall option) internal GPS which helps avoid any external nav errors, but this option can still use any external input for comparison.
Later modifications have added Geometric Altitude, only available with GPS; other options included peaks mode and obstacle database – an invaluable tool if updated regularly (it was free).

AFAIR the early A320s were equipped with the Honeywell EGPWS, but later aircraft may have the T2CAS (TAWS) from a different vendor. There are differences between eash systems' inputs and with computation, but both meet the same operational requirements spec.

[catiamonkey]

I am not seeing how a non-gps A320 could have EGWPS...

http://www.skybrary.aero/index.php/A...ia,_2003_(CFIT)

The aircraft received erroneous information from the ADS VOR which was fed to the flight deck VOR display, the FMS, the navigation displays and the EGPWS computer with its associated Terrain Awareness Display (TAD). A single common position source error thus adversely affected all these apparently independent navigation/situational awareness systems. The existing certification standards for the aircraft navigation systems were met but were not sufficient to protect against this problem.

[underfire]

Okay, I understand..

Read right thru aircraft vs system, and systems before Geometric Altitude.

So while the EGPWS had its own GPS, the aircraft did not...got it.

As SafetyP notes, the peaks/obstacle database is incredibly valuable...

[catiamonkey]

No, there is NO requirement that a EGPWS unit have any sort of GPS. Quoting from the report I linked to:

The certification of the TAWS aircraft installation was guided by the JAA document “Section One: General Part 3: Temporary Guidance Leaflets LEAFLET NO 12: Certification Considerations for the Terrain Awareness Warning System: TAWS.” – referred to as TGL12. This allowed the use of the aircraft navigation system, which was designed for area navigation, to be the source of positional information for the TAWS. The capabilities of such area navigation systems vary from very poor to very good. There are no minimum positional accuracy requirements imposed on the source of positional information for TAWS, other than by reference to relatively relaxed area navigation requirements, and no requirements to supply TAWS with relevant indicators of navigational data quality. There is no requirement to have GPS as a source of positional information for the TAWS. The standard GPWS alerts, pre-TAWS, are viewed as fall back protective modes in the event of a failure of the new TAWS database based modes.

[discus2]

Very interesting thanks guys.
Are the EGPWS alarms triggered when the gear/flaps aren't selected or when they aren't actually set in the landing configuration then ?