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?