Class A - En Route — Page 457, Lesson 564

We're now in the en-route phase, and I want to walk you through two critical emergency procedures: the rapid descent and the engine failure drift down. Let's start with the rapid descent. The procedure involves configuring the aeroplane for the maximum rate of descent. Now, to achieve a maximum rate of descent, you need the excess power required to be as large as possible. Let me unpack that. In level flight, thrust equals drag. If you want to descend, you reduce thrust, so now drag exceeds thrust, and that difference is the excess power required. The bigger that difference, the faster you descend. So to make that difference as large as possible, drag must be high and speed must be high. So the first actions of the pilots are to don the oxygen masks, close the throttles, apply the speed brakes, and then lower the nose to allow the aeroplane to accelerate to maximum operating speed, which is either VMO or MMO. VMO is the maximum operating speed expressed in knots indicated airspeed, and MMO is the maximum operating speed expressed as a Mach number. This configuration is then maintained until at least 10,000 feet, or the minimum safe en route altitude, where there is sufficient oxygen to breathe. That's the key point — the whole reason for the rapid descent is to get down to an altitude where the air is dense enough to sustain you without supplemental oxygen. Now let's move to the engine failure and drift down. In the case of an engine failure during flight, the remaining thrust is no longer sufficient to balance the drag force, and therefore the cruise speed cannot be maintained. The only solution is to descend to a lower flight altitude, where the remaining engine can provide enough thrust to balance the drag and allow level flight once more. To achieve this, the aeroplane is initially flown level to allow the aeroplane to decelerate from the cruise speed to the velocity of minimum drag, which we call VMD. At VMD, the nose is lowered to maintain VMD, which can now be thought of as the "speed for minimum excess drag," as shown by Figure 17.11. Let me explain why VMD matters here. At VMD, the drag is at its minimum, so the excess drag — the difference between drag and the remaining thrust — is also at its minimum. That means the aeroplane descends at the shallowest possible angle, giving you the maximum distance for the altitude you lose. That's exactly what you want after an engine failure, because you need to clear obstacles and reach a suitable landing area. As the aeroplane descends into the lower atmosphere where density is greater, the remaining engine can develop more thrust, which will eventually equal drag. This is the GROSS level-off altitude, but it would give no performance margin. So the DRIFT DOWN PROCEDURE is continued to a lower altitude, the NET level-off altitude. Let me make that distinction clear. The gross level-off altitude is where the remaining engine's thrust just balances drag, but there's zero margin — any slight disturbance and you'd start descending again. So you continue down to the net level-off altitude, which gives you a safety margin. Figure 17.12 is a graph which allows flight crew to determine distance flown, and gross altitude, following engine failure. The current lines are the drift down profiles for various aircraft weights. So you enter the graph with your aircraft weight, and you can read off how far you'll travel and what gross altitude you'll reach. This procedure is called the drift down, and it produces a drift down profile. This path must, of course, be above all relevant obstacles, but that will be discussed later. Let me just tie it together. The rapid descent is for when you need to get down fast — think cabin depressurisation. The drift down is for engine failure, where you descend slowly at VMD to maximise distance. Both are about managing the relationship between thrust, drag, and altitude. In the rapid descent, you want maximum excess drag to descend fast. In the drift down, you want minimum excess drag to descend slowly and cover maximum distance.