View Full Version : GPWS and EGPWS curiosity

8th Aug 2005, 07:32
Just been thinking about how GPWS or for that matter EGPWS is disabled (read some comments on here a while back). When an aircraft is configured for landing is the GPWS disabled automatically so not to make nuisance alerts??. Does this happen with EGPWS??

I fly VFR but my GPS terrain warning beeps off when about to land (1000ft AGL) and is annoying but I can hit ENTER and it stops.

My concern is that say in a case like Lockhart River if either device detected a terrain issue would the pilot always be alerted. If say the gear going down disables the alerts what is the good of it, unless it has to be disabled by the pilot himself.

Any explanations would be welcome.

J :ok:

Fred Gassit
8th Aug 2005, 07:42
There are a bunch of different warnings you can get, only a few can be inhibited, "hard" warnings like "Terrain Terrain Pull up" cant be inhibited, a commonly overridden one is "too low flaps" when doing a flapless landing for example.

Speeds high
8th Aug 2005, 08:23
With the gear down the GPWS thinks the aircraft is going to land, and therefore automatically inhibits the "Terrain Terrain Pull up" warning, however if terrain closerure rate is excessive will give a "Sink Rate" warning instead.

I question i have on EGPWS, is does it include in its database the position of the runway i.e. if the aircraft is going to land short will a EGPWS give a warning of this?

8th Aug 2005, 08:42
What appears to be used less and less, and what should give the loudest warning of the lot, is the computer between one's ears.


8th Aug 2005, 09:27
EGPWS provides Terrain Clearance Floor Protection for aerodromes that meet rwy type and length criteria. Have a look under Terrain Clearance Floor at http://www.egpws.com/general_information/videos.htm

8th Aug 2005, 10:41
Predictive features of the EGPWS can be overridden by use of the TERR SYS OVRD button or by a system failure. TERR SYS OVRD may be selected on an approach to a runway that's not in the EGPWS database which would return a warning as the aircraft would think it's flying towards the ground not realising there is a runway there.

In either of these cases little or no warning may be given of impending terrain impact if the aircraft is being flown towards the ground in a stable approach configuration and there is no ILS glideslope available.

(paraphrased for brevity from the OM).

8th Aug 2005, 12:31
EGPWS modes are varied and can be quite daunting to comprehend. BOSTON has given you the best starting point for understanding it, the videos are very functional and assist in understanding the concept. I will attempt to explain it while keeping it as simple as I can. There are 6 basic modes that provide reactive warnings based on inputs of Radio Altitude, IAS and Vertical Speed (amongst others) to the GPWS, and then the predictive functions of the EGPWS take into account GPS position relative to a data base of terrain, obstacles and runways.

TCF (Terrain Clearance Floor) and RCF (Runway Clearance Floor) provide protection around a data base aerodrome. Provided the aircraft remains above the TCF envelope for the runway in the data base , warnings are not triggered. However the GPWS will still provide annunciations/warnings in MODE 1 excessive vertical speed, MODE 2 Excessive Terrain Closure Rate, MODE 3 Alt loss after T/Off, MODE 4 Unsafe Terrain Clearance and MODE 5 Glide Slope Deviation. On top of this the TCF is constructed to provide a stepped protection area that would be detailed on BOSTONS link to the Honeywell site. Simplistically put, it slopes from the runway elevation to 400' at 5nm. (Approximate) If you descend below this envelope you will still get a GPWS warning.

There certainly is an inhibit function for the EGPWS (Terrain Inhibit) that prevents spurious warnings on approach to an aerodrome not held in the data base. Hence the aircraft flight manual will usually if not always require that terrain inhibit be selected when on approach to an aerodrome not contained in the data base (A/D's less than 2000'). But even with this function selected the GPWS basic MODES 1 through 6 will still be operational.

Crew also have the ability to select GPWS flap override, this is one method of inhibiting part of mode 4 to have the GPWS ignore flap setting if a flapless approach is required, however mode 4 will still be operational in its other functions eg. TOO LOW GEAR.

Selecting the gear down also satisfies part of the Mode 4 function, if it didnít then you would receive spurious warnings on approach below 500 feet AGL.

I couldnít possibly hope to detail the full working and modes of the EGPWS/GPWS here, but I hope the brief summary from my failing intellect helps a little.

8th Aug 2005, 21:50
Thanks to Splat DA & Boston, now this all leads me to the next question. After watching all the debate over new requirements for EGPWS and TAWS rather than the old GPWS it seems to me that while they are a great tool and in many cases prevent a CFIT in a case such as the Lockhart River crash and Benalla the extending of gear and flap partially inhibits the tool. This is fine if you are definitely in the right position, however if you are not where you think you are and ignore or cancel or have suppressed warnings you could be in for a big prang.

Is this the case or have I missed something here?

I know what "Prospector" is thinking, and he's right, make sure you are where you should be, but too may are relying on systems to protect them when they will not and can not it seems.

Enjoying the constructive comments, keep it coming.

Cheers J:ok:

9th Aug 2005, 00:41
Speeds high

With the gear down the GPWS thinks the aircraft is going to land, and therefore automatically inhibits the "Terrain Terrain Pull up" warning, however if terrain closerure rate is excessive will give a "Sink Rate" warning instead.

Can you please expand on the above? Please excuse my ignorance but I'm a very steep learning curve and as I don't fly and have enough trouble operating my mobile telephone all this technical information tends to take along time to sink in. Plus every pilot I talk to seems to have a different interpretation, even those in airlines.

Do you mean if you're descent rate is excessive you will receive a "Sink rate" warning?

Fiona Norris

9th Aug 2005, 06:19
UK CAA has a good guide to the limitations of GPWS -its why we're all moving to EGPWS.

9th Aug 2005, 07:40

Thanks for the link. It seems that there has been at least one case (according to the CAA) of crew muting the alert only to find it was alegitimate one.

However I am still interested to know if in fact this muting can also be a function of gear and flap down situations. Can anybody answer this? And does it also apply to EGPWS.

I see my thread has caught the eye of Lisag and no doubt this is of interest to her and anybody else related to similar accidents.

What I think could be an issue is ignoring or muting or treating warnings as a nuisance when still in IMC this can be very dangerous, fine if you have become visual but if you are not how can you be sure unless you find out the hard way.

Surely some of my fellow Queenslanders who fly the regional turbo props have an answer and an opinion, as this is what they work with every day and may depend on one day too.



9th Aug 2005, 09:42
Thank you for starting this thread. I appreciate your interest in the GPWS which was equipped on the Metro at LR.

Okay to put it in plain English, for me a dope, who doesn't fly who now reads way too much information on GPWS and EGPWS but still can't quite figure out all this technical jargon.

I have been told that if the plane was configured to land, gear down, stabilised approach, that the GPWS would not alert the pilots of the rising terrain just in front of them?? But the EGPWS which looks forward would have alerted them to the rising terrain?

Fiona Norris

9th Aug 2005, 09:52
Hello all

My tupence worth (for what it's worth)

Both EGPWS & GPWS warning modes do change significantly depending on the gear and flap status of the aircraft.

Neither of them want you to go near the ground with flaps in landing position and the Noise Suppressing Rubber thingies down.

Either system is expensive to fit to an aircraft ( the expense varies between spectacular and obscene) so the only aircraft that have a GPWS/EGPWS system are likely to be the ones that have to have one.

This means that not only will it be mandatory to have one but also to tell pilots what to do if it talks to you. So you will find...

If the chap hops out of the cupboard, you are obliged to heed his warning unless you are in VMC and it is daytime and you are very, very sure he is telling lies. If you are IMC or it is dark, off you go. (and you will be trained to do this)

There are times & circumstances when you can inhibit warnings. There are other times when you can disable all or part of the system (various cunning switches with guards over them etc, or in the older ones something as barbaric as pulling CB'S)

only as specified in the approved operations manual children, never never try this at home!!!

At the end of the day, you can't beat knowing where you are, but you must never ignore the warning if it comes when you can't see!!

Don't know if this has helped at all but ..... who knows

Cheers and have fun


Lisag, just saw your last post.

With all the usual riders about not knowing all the facts of the accident (Thoughts with all of them) ..........

Yes EGPWS may have saved the day, whereas GPWS thought he was landing and didn't mind that the ground was rising up. In the absence of a Glide Slope it would not have known whereas EGPWS has a terrain and runway database so it will (normally) tell you if you are trying to land where you shouldn't.

Bad things have been known to happen though, so again best to be sure of where you are.



Roger Standby
9th Aug 2005, 10:25
"Pull up, Pull up"

"Shutup, Gringo!"

Cloud Cutter
9th Aug 2005, 11:06
Ha ha, that was classic (overlooking the fact that people died). I guess 'Gringo' got his revenge!

9th Aug 2005, 11:35
Now that Lisag has entered the debate and I hope she does not mind me using the Lockhart River case as an example, but here goes.

If you flew A. GPWS and B. EGPWS equipped aircaft deliberately or otherwise toward steep rising ground as was the likely course of the Metro, in the landing configuration would you expect to get a PULL UP warning or any of the other modes.

Would you get a different warning if the gear and flaps were up?

My suspicion is you may not get any warning at all untill its too late with the old GPWS as the rise in ground height is rapid but I am not sure. I expect that EGPWS will always give a pull up regardless of the landing config.

Going to watch a shuttle do a dead stick landing... I hope! Look forward to a response.

Blue skies and :D


9th Aug 2005, 12:25
J430 and Lisag.

I might be able to answer your queries from another direction -- a former topend Metro 23 pilot, and friend of Hermie's.

"Turning off" a GPWS or EGPWS.

It was a legal requirement that the M23 dispatch that day with a serviceable GPWS in accordance with Civil Aviation Order 20.18 subsection 9.

That particular aeroplane had been fitted with a 'straight' GPWS unit prior to 1 January 2001, therefore the owner/operator was within the 5 year 'amnesty' before that aircraft required fitment of an EGPWS -- GPWS with predictive terrain warning via an onboard terrain database and GPS navigation information.

So it was legal, and the unit had to be working before dispatch that day.

The manufacturer's operations manual describes the functional check required each day of the GPWS. One of the rostered pilots, probably the FO, would have this check as part of his cockpit preparation duties. It involved pressing the glareshield annunciator until the cockpit speaker announced "Glideslope" then "Whoop Whoop Pull Up" three times with the "PULL UP" annunciator flashing to indicate a successful test.

There are 9 modes of alerting in a 'straight' GPWS, modes 1, 2A, 2B, 3, 4A, 4B, 4C, 5 and 6, all well explained in the videos and downloads from www.egpws.com

Can you turn them off, like J430's gps unit's warning? That is a fair question.

You can cancel a warning for only one of those nine modes; the Mode 5 Descent Below Glideslope warning that is only active if you are flying an ILS instrument landing system approach. Not applicable that day - there is no ILS at Lockhart River.

You can only disable one of those nine modes: Mode 4B, Insufficient Terrain Clearance, Flaps Up. This is done by lifting the guard over the relevant switch, and depressing the switch button. This enables you to land without setting the normal landing flap, something you might wish to do if you had a flap malfunction.

The Metro 23 has four flap selections available. Up, one quarter flap, half flap, and full (landing) flap.

The manufacturer operations manual instructs pilots to configure to propellor synch off, speed levers high, flap one-quarter, and gear down in the approach or landing circuit.

There is a specific instruction to not select any more flap until landing is assured.

There is some discussion in the Metro 23 pilot fraternity about this, as it is difficult to keep a Metro 23 within the Category B circling area (radius of 2.66nm around the landing runway) with only one-quarter flap, and probably doing less than 190 knots.

However the GPS NPA at Lockhart River is a runway aligned approach, and I would expect that the approach was flown with one-quarter flap until the runway was sighted.

In any case, they would not have got a Mode 4B "Too Low Terrain" warning until they were around 200 to 500 feet above the ground. Note also that Mode 4B just detects a minimum height for the current configuration, no monitoring of descent rate or terrain closure rate.

They did have protection against Excessive Descent Rate (mode 1) and Excessive Terrain Closure Rate (mode 2A).

They could not turn those off, and they had to be working every time they dispatched.

You might still have protection via Mode 6, Altitude Callouts. Altitude callouts are actually Radio Altimeter Height callouts, where a synthesized voice announces the height above the ground that you have just descended through.

In the Metro 23's I flew, the Mode 6 was only programmed to call out "Minimums" when the Radio Altimeter was set to an approach minimum. The jet I fly now has the 500', 100', 50-40-30-20-10 for all approaches, plus the "Minimum" call for instrument approaches.

So maybe they didn't have a mode 6 callout to alert them. In other words, they might not have been turned on. There was probably no requirement for them to be turned on.

But don't forget - the Excessive Descent Rate and the Excessive Terrain Closure Rate modes were on and presumably functioning. You can't turn them off easily, maybe circuit breakers could be pulled, but that is unlikely.

At the approach speed they were doing, they would have needed to have had....

.... an excessive sink rate, in the order of 2,500 feet per minute, to set off the Mode 1 warning, or

.... an excessive terrain closure rate, greater than 2,200 to 3,600 feet per minute, to trigger a Mode 2A warning.

2500 feet per minute is a vertical speed of 45km/h or 12m/s, independent of your groundspeed, you descending to meet the terrain or the terrain coming up to impact you.

Should you actually get one of these warnings, you need to take immediate action to establish your best angle of climb.

It might be possible to immediately pull up, apply full power and start motoring up at 3000 feet per minute.

But that might not be enough.

An aeroplane that is doing 150-180 knots (the go-around and clean climbout speeds for a M23)and can climb at 3000 feet per minute, is climbing at a gradient of about 16%.

If the hill in front of you has a gradient of 17% and is higher than you, you might hit it.

If you had only a straight GPWS, and were approaching a hill with a very steep slope, you may get little or no warning. The GPWS input is your radio altimeter, which looks down, not ahead.

The 'look ahead' function is the Enhanced part of the EGPWS, which has a knowledge of where it is via its own GPS, and a knowledge of the terrain on the surface of the earth via its terrain database.

So the big advantage of EGPWS is its knowledge of the terrain around you. "Look ahead" is a bit of a misnomer though; it does not 'see' the terrain, it just knows it is there.

An EGPWS might have made a big difference that day. Might have, we don't know for sure.

Is that helpful?

9th Aug 2005, 13:00
Thanks for the detailed reply. The answer I was after is no, landing config does not influence alerts as such.

As for Lisag's interest I am sure you have helped her understand it better too.

My next question and probaly should be another thread is how can two GPS aircraft, both regurly flown on the same track by the same crew get themselves too dead! (L/River and Benalla)

I have raised electronic interference here before and not given a warm reception, however I know first hand that phones affect transponders (similar bands) and maybe some GPS receiver can be too.... Maybe its time to experiment.

I have no vested interest in either accident, but I just can not get over some similarities between the two.

I like my VFR.....

Thanks again.

J:ok: ;)

9th Aug 2005, 14:35
f you flew A. GPWS and B. EGPWS equipped aircaft deliberately or otherwise toward steep rising ground as was the likely course of the Metro, in the landing configuration would you expect to get a PULL UP warning or any of the other modes.

Would you get a different warning if the gear and flaps were up?

Rough and ready answers?
A. Probably not. If the hill was much higher than you, and of a steep gradient, you might get insufficient warning or nil.
B. In most cases, yes.

Clean config or landing config make a difference? No. What you are talking about here is rate of closure, a Mode 2 warning. You would get a terrain closure warning ("WHOOP WHOOP Pull Up") well before you got to the 245'-1000' radio altitude that would trigger the unsafe terrain clearance mode 4 warnings.

On the topic of equipment failures and interference....

One of the things that you learn (or have beaten into you!) as you proceed through life as an IFR pilot, is that the information you need is duplicated many time over. You never rely simply on one source of information, or on one gadget to get you through a situation.

This is the state of mind and professional approach that IFR operators try to develop in their pilots.

For example, fuel calculations. Never just look at your gauges and say, great, I have 4 tonnes left in the tanks. You recall that you started with 9 tonnes. You then scan the fuel used totalizers and add them up. 5 tonnes burnt. 4+5=9. You start to feel reassured. You then think, departure was 2h15m ago, at just under 2 tonnes/hr, plus around 500kg for takeoff and climb to cruise.

One observation and two calculations to back it up as a check of its reasonableness. Why? Because fuel quantity gauges are gadgets, and gadgets sometimes malfunction.

How about an approach like an ILS?

Well first, you need to have those needles centred as you fly down. But it is best to start by flying over the locator beacon at a fixed altitude, normally 3000' fly level and intercept the glideslope from underneath. That way you have flown station passage straight over a known point at a known altitude and minimize the probability to capturing a false localiser course or a false glideslope. Gadgets. The glideslope will start down at a certain DME or GPS distance from the aerodrome. Crosscheck. In my Cat C aeroplane that flies down the ILS at around 125kts, I know that I will stay on slope if I have a rate of descent that is roughly 5x my groundspeed, about 650 to 700 feet per minute. I will know that I have set the correct QNH when I cross the outer marker at the outer marker check height. I know I have the right outer marker because I know it is located at 4.7DME. The GPS in the corner of my eye also says 4.7nm to run. When I hear the GPWS call out "Five Hundred" at 1.6DME that is good because that is the DME distance I should have a radalt of 500 and be on glideslope. I will know that I have the gear down and the landing flap set not only because the three green lights are glowing and the flap indicator show landing flap set, but also the aircraft is in a 2.5 degree nose down pitch attitude holding the reference speed plus 5 at a power setting of roughly 63% N1. All in the ballpark for a landing at 31 tonnes LW.

Lots and lots of information and knowledge, each little piece checked against another to confirm that I am where I am supposed to be. Any discrepancy is a niggly loose end, to sound a warning - 'what is wrong with this picture?'

And then there is a more experienced guy sitting to my left with the same questions running through his head, observing the same little bits of information and looking for the odd man out.

The point of all this?

1. Gadgets fail. The best of them do. Pilots love their gadgets. They use their gadgets. But a wise pilot never TRUSTS a gadget! Cross check everything one gadget says with another gadget, or the evidence of your eyes, or your quick calculations on your whiz wheel or calculator.

2. Passengers want safe pilots that look at everything when they board an aeroplane. Pilots need a working environment that supports them in gaining these skills, supports them in approaching their work with a critical and disciplined method. They dont want pilots that are struggling to get decent sleep, living near the poverty line, encouraged overtly or otherwise to be yes men, toeing a company line to get the job done. These skills are what brings out all the potential of the gadgets. It takes time, a lot of time, to get it together with all of the skills you need. Unfortunately as soon as these skills are gained they are lost to GA, because who wants to work so hard in such a terrible employment situation?