Ground Proximity Warning System — Page 461, Lesson 559

We're moving into the next set of GPWS modes now. We've covered the terrain-based warnings, and now we're looking at the approach-specific alerts. Let's start with Mode 5. Mode 5 is the Below Glide Slope Deviation Alert. Its job is to warn the crew when the aircraft descends more than 1.3 dots below the ILS glide slope. That's the trigger point — 1.3 dots of downward deviation. The alerting envelope has two distinct areas: soft and hard. In the soft area, you get a reduced-volume voice message of just "GLIDE SLOPE." In the hard area, which begins at approximately 2 dots below the glide slope, you get a full-volume "GLIDE SLOPE, GLIDE SLOPE" — the message is repeated. And in both areas, the voice message repetition rate increases as the glide slope deviation increases and as the radio altitude decreases. So the worse the deviation and the lower you are, the more insistent the alert becomes. Now, arming conditions for Mode 5. The mode is armed when three things are true simultaneously: a valid signal is being received by the glide slope receiver, the gear is down, and the radio altitude is 1000 feet or less. There's also a crew control. The mode may be cancelled or inhibited by pressing either pilot's BELOW G/S light while below 1000 feet radio altitude. That's the visual annunciator — the BELOW G/S light — and pressing it acts as the inhibit switch. The mode will re-arm automatically when the aircraft climbs back above 1000 feet radio altitude. One important priority note: Modes 1 through 4 aural alerts and warnings have priority over Mode 5 aural alerts. However, both the PULL UP and the BELOW G/S lights could be illuminated at the same time. So the aural priority doesn't prevent simultaneous visual indications. Let me summarise the Mode 5 annunciations. The aural alert is "GLIDE SLOPE," and the visual is the BELOW G/S light, which is press-to-inhibit. Now let's move to Mode 6A — Below Selected Minimum Radio Altitude. This mode provides an aural alert if a descent is made below the minimum decision altitude cursor in the radio altimeter. That cursor is the adjustable bug on the radio altimeter that you set to your decision altitude. Mode 6A operates between 50 and 1000 feet of radio altitude — that's its active envelope. This alert is aural only, and it consists of "MINIMUMS, MINIMUMS" sounded once. The mode is re-armed when the radio altitude becomes greater than that selected with the altitude cursor. So once you climb back above your selected minimum, the system is ready to alert again on the next descent. Now Mode 6B — Altitude Call-outs and Bank Angle Alert. This mode provides call-outs of selected altitudes and minimums. The specific call-outs used are a customer option — meaning the airline chooses them — but for example, they may consist of calls at 200 feet and 100 feet to decision height, or absolute height call-outs from the radio altimeter with respect to the ground. So either relative to decision height or absolute height above the terrain. Finally, the Bank Angle Alert. "BANK ANGLE" can be used to alert crews of excessive roll angles. This is a crucial point: the bank angles are not an aircraft manoeuvre limit. They are a limitation decided upon by the airline. However, the limit reduces with proximity to the ground. Why? Because of the reduced wing tip clearance — the system is designed to prevent wing tip or engine damage during take-off and landing. So as you get closer to the ground, the allowable bank angle tightens to protect the wing tips and engines from striking the surface. That's the complete picture for Modes 5, 6A, and 6B.