Class A - En Route — Page 457, Lesson 564

Let’s pick this up right where the emergency descent left off, because the next part of the chapter is a natural follow-on: what happens when you lose an engine in cruise. First, a quick recap of the emergency descent we just covered, because it sets the scene. That procedure was about getting down fast when you have a pressurisation problem. The pilots configure the aeroplane for the maximum rate of descent. To get that maximum rate, the excess power required has to be as large as possible, which means drag must be high and speed must be high. So the first actions are: don the oxygen masks, close the throttles, apply the speed brakes, and lower the nose to accelerate to the maximum operating speed — that’s either VMO, the maximum operating speed in terms of indicated airspeed, or MMO, the maximum operating speed in terms of Mach number. You hold that configuration until at least 10,000 feet, or the minimum safe en route altitude, whichever is higher, because that’s where there’s enough oxygen to breathe without the masks. Now, engine failure and drift down. This is a different scenario entirely. Here, you’ve lost an engine in flight, and the remaining thrust is no longer sufficient to balance the drag force. That means the cruise speed simply cannot be maintained. The only solution is to descend to a lower flight altitude, where the remaining engine can produce enough thrust to balance the drag and allow level flight once more. Let me walk you through the mechanics of how that descent is flown, because it’s quite elegant. The aeroplane is initially flown level to allow it to decelerate from the cruise speed down to the velocity of minimum drag — that’s VMD. At VMD, the nose is lowered to maintain that speed. Now, here’s the key insight: at this point, VMD can be thought of as the “speed for minimum excess drag.” Why? Because at minimum drag, the excess drag — the drag that the engine has to overcome beyond what it can produce — is minimised, so the aeroplane descends at the most efficient rate for the thrust available. As the aeroplane descends into the lower atmosphere, the air density increases. That’s the crucial point. The remaining engine can develop more thrust as density increases, because the engine produces thrust based on the mass of air it processes. Eventually, that thrust will equal the drag. That point is called the GROSS level-off altitude. But — and this is important — the gross level-off altitude gives you no performance margin. It’s the theoretical point where thrust exactly equals drag, but there’s no buffer for any error, any wind, any manoeuvre. So the DRIFT DOWN PROCEDURE is continued to a lower altitude, which is the NET level-off altitude. That’s the altitude you actually plan to fly at, with a safety margin built in. Now, there’s a graph — Figure 17.12 — which allows the flight crew to determine the distance flown and the gross altitude following an engine failure. The curved lines on that graph are the drift down profiles for various aircraft weights. So for a given weight, you can trace the drift down path and read off how far you’ll travel and what altitude you’ll reach. Let me also point you to Figure 17.11, which shows the changes to thrust and drag after an engine failure. It illustrates exactly what I’ve been describing: the thrust line dropping, the drag line staying, and the point where they cross being your gross level-off altitude. This whole procedure is called the drift down, and it produces a drift down profile. That path must, of course, be above all relevant obstacles — terrain, mountains, anything in your way — but we’ll get to that obstacle clearance discussion later in the chapter. So, to tie it together: emergency descent is for pressurisation, fast down, high drag, high speed. Drift down is for engine failure, slow down to VMD, descend to where the remaining engine can hold level flight, and plan to stop at the net level-off altitude, not the gross one, because you need that margin.