GPWS: the CPL Tech Knowledge item that keeps showing up on KDRs
Every time someone runs their Knowledge Deficiency Report through our KDR Refresher, we get a bit more of a picture of where CPL candidates are dropping marks. For Aircraft Technical Knowledge, one code is well out in front: 26.62.2, "Describe the function and operation of a typical GPWS system." It's turned up on 46.8% of the CPL Tech reports that have gone through the tool. The next code down is at 36.2%.
That's a lot of people losing a mark on something that isn't actually complicated. So here's what's in the refresher for 26.62.2, with a bit more explanation around it.
What the system is for
GPWS stands for ground proximity warning system. Its job is to reduce controlled flight into terrain, which is the polite name for flying a perfectly good aeroplane into a hill. It does that by warning the crew when the aircraft is in, or getting close to, an unsafe terrain clearance situation.
Two words in that sentence matter for the exam: it warns. It doesn't fly the escape for you and it won't stop you hitting the ground. When it goes off, you fly the warning response procedure for that aircraft and operator, which usually means power up, pitch up, wings level, and keep climbing until it stops talking. Have a look outside later.
Basic GPWS
The original GPWS works mostly from the radio altimeter (your actual height above the ground under you) along with aircraft configuration, so gear and flap position. Depending on the fit it also takes barometric altitude and vertical speed, and glideslope deviation from the ILS.
It compares those inputs against a set of modes. Each mode covers one way of getting into trouble:
Mode 1: excessive descent rate. Coming down too fast for how close the ground is.
Mode 2: excessive terrain closure rate. The ground is rising towards you faster than it should be.
Mode 3: altitude loss after take-off or go-around. You've started sinking when you should be climbing.
Mode 4: unsafe terrain clearance when you're not in the landing configuration. Low, with the gear or flaps still up.
Mode 5: excessive deviation below the ILS glideslope.
If you can list those five and say what each one is looking for, you've got the "operation" part of 26.62.2 covered.
Enhanced GPWS / TAWS
The problem with the basic system is that a radio altimeter only sees straight down. It has no idea what's in front of you, so with steeply rising ground the warning can come very late.
Enhanced GPWS fixes that by adding the aircraft's position (GPS, or fed from the FMS) and a terrain and obstacle database. Now the system can look ahead along your track and warn you about the ridge that's still a minute away, and it can show the terrain on a display. ICAO's generic term for this is Terrain Awareness and Warning System, TAWS. In practice EGPWS and TAWS mean the same thing, and the old radio altimeter only version gets called "basic GPWS" to keep them apart.
The short version
This is more or less the exam tip straight out of the refresher: GPWS warns of terrain and clearance threats. Basic GPWS is radio altimeter and configuration based. EGPWS/TAWS adds position and a terrain database.
Learn the five modes, know what EGPWS adds and why, and know that the pilot flies the escape. Do that and 26.62.2 shouldn't be on your next KDR.
If you've sat CPL Tech recently, run your KDR through the refresher. It points you at the exact syllabus items you missed, and every report that goes through it sharpens up the picture for the next person.
