Ground Proximity Warning System — Page 461, Lesson 559

I want to walk you through Mode 5 and Mode 6 of the Ground Proximity Warning System. We've covered the earlier modes, so now we're looking at the ones that protect you during the approach and landing phase. Let's start with Mode 5 — the Below Glide Slope Deviation Alert. This mode watches your position relative to the ILS glide slope. It alerts the flight crew when you descend more than 1.3 dots below the glide slope. That's the trigger point — 1.3 dots of downward deviation. The alerting envelope has two areas: a soft area and a hard area. In the soft area, you get a reduced volume voice message of just "GLIDE SLOPE" — one call, quieter. In the hard area, which is approximately 2 dots below the glide slope, you get a full volume "GLIDE SLOPE GLIDE SLOPE" — the call repeated at full volume. Here's an important detail: in both areas, the voice message repetition rate increases as the glide slope deviation increases and as the radio altitude decreases. So the worse your deviation and the lower you are, the more urgently the system calls out. Now, when is Mode 5 armed? Three conditions must be met simultaneously. First, a valid signal must be received by the glide slope receiver. Second, the gear must be down. Third, the radio altitude must be 1000 feet or less. All three together arm the mode. You can cancel or inhibit Mode 5 by pressing either pilot's BELOW G/S light while below 1000 feet radio altitude. That's the visual annunciator — the light labelled "BELOW G/S" with "P TO INHIBIT" — pressing it inhibits the mode. The mode will re-arm when you climb back above 1000 feet radio altitude. One more priority note: Modes 1 through 4 aural alerts and warnings have priority over Mode 5 aural alerts. However, both the PULL UP and BELOW G/S lights could be illuminated at the same time. So the voice calls from the earlier modes take precedence, but the visual warnings can coexist. Now let's move to Mode 6A — Below Selected Minimum Radio Altitude. This mode provides an aural alert if you descend below the minimum decision altitude cursor in the radio altimeter. That's the bug you set on the radio altimeter. This mode operates between 50 and 1000 feet of radio altitude — that's its operating envelope. The alert is aural only — no visual warning. It consists of "MINIMUMS, MINIMUMS" sounded once. The mode is re-armed when the radio altitude becomes greater than the altitude you selected with the cursor. So once you climb back above your selected minimum, it's ready to alert again. Finally, Mode 6B — Altitude Call-outs and Bank Angle Alert. This mode provides call-outs of selected altitudes and minimums. The specific call-outs are a customer option — the airline chooses them. For example, they might 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. Mode 6B also includes the "BANK ANGLE" alert, which warns crews of excessive roll angles. Here's a critical distinction: the bank angles are not an aircraft manoeuvre limit — they're a limitation decided upon by the airline. However, the limit reduces with proximity to the ground due to the reduced wing tip clearance. The purpose is to prevent wing tip or engine damage during take-off and landing. So to summarise the key contrasts: Mode 5 watches glide slope deviation with soft and hard alert areas, Mode 6A watches your selected minimums with a single aural call, and Mode 6B gives you customer-selected altitude call-outs plus a bank angle alert that tightens as you get closer to the ground.