DRIFT DOWN TO A STABILISING ALTITUDE AFTER ENGINE FAILURE

With thanks to SFO Peter Kohn (BA LHR - A320 fleet) 19.5.1 Introduction to the problem

In the event of an engine failure in the cruise, the aircraft may too heavy to be able to maintain the cruise Flight Level even with continuous thrust set. It will commence a drift-down to a levelling-off altitude which may be BELOW MSA for the route sector ahead. .

Data presented by the aircraft computer and / or extracted from the QRH includes target drift-down speed, stabilising altitude, plus distance and time to get there. These vary according to starting altitude, aircraft weight, ISA deviation and air bleed demands from packs and ant-ice. Wind component must be considered, to translate this into a ground distance. Engine-out predictions calculated by on board flight computers (FMC on Boeing / FMGC on Airbus), will incorporate all of these factors. Fight crew drift down tables or graphs in the performance manual (often repeated in the QRH for rapid access) will usually require reference to a separate chart to incorporate wind component and convert air distance to ground distance.

Graphs are easier to interpret readily but not all manufacturers subscribe to that point of view. Some produce tables instead. Examples of both presentations are offered below for your consideration.

19.5.2 Initial Actions 1. Fly the aircraft.

Set power on the remaining live engine to Maximum Continuous Thrust (MCT).

Re-trim the rudder to fly straight. Some aircraft (e.g. Airbus) will auto-trim to maintain balanced flight so negating the need to disconnect and trim the rudder manually. However if disconnecting the auto pilot is required, or has occurred due to aircraft system failures, do be aware that rudder is extremely powerful at high TAS.

Allow the speed to decrease to the drift-down value displayed in the FMC / FMGC CRUISE page, or as extracted from the QRH and then maintain it. On Airbus aircraft, this speed is permanently displayed on the ASI speed ‘tape’ as a green dot. Regardless of where or how it is displayed for your type, the aerodynamic significance of this speed is that it gives the highest Lift / Drag ratio. Therefore it is also the optimum speed to fly for best glide range in the event of loss of ALL engines.

In this configuration (MCT & recommended speed) the drift down will provide maximum range and minimum fuel burn.

2. Deal with the failure:

Initiate the appropriate checklist for the failure condition. It may be that a vibrating engine may run smoothly when throttled back. Even though you must consider it failed for performance purposes, you may wish to keep it running to maintain its ancillary services and systems redundancy. Carry out the checklist using the aircraft electronic checklist or appropriate Quick Reference Manual/Handbook (QRM/QRH), as dictated by your SOP.

In reality these initial actions will run in parallel: The aircraft’s deceleration towards target descent speed at MCT might take a minute or so, providing time to identify the failure as a crew and to initiate the abnormal drill from the second pilot.

E.g. A short-haul Airbus at typical weights takes around 90 seconds to reduce to drift down speed and then will descend at an initial ROD of around 300 fpm. This should provide ample time for the crew to apply correct, methodical initial actions, initiate any drills associated with the failure and to make a radio call. The subsequent descent rate is not necessarily an alarming one.

19.5.3 Subsequent considerations.

After the initial actions to keep the aircraft flying (vertical profile) and to initiate any abnormal drills required, the problem becomes one of which way to point the aircraft (lateral profile).

a. Obstacle Strategy (Terrain-Critical: Drift-down to maximum possible stabilising altitude)

If stabilising altitude is below the MSA, then assess the time and/or distance needed to drift-down stabilising altitude. Is it safe to continue or is a turn back is required?

Figure 19.5.4 shows how altitude lost per track mile is not a linear relationship: The initial rate of descent will be higher than the final one, which will reduce until it eventually reaches zero as the curve flattens out at the stabilising altitude. In reality we never get a type specific representation of this curve. We only know that that our aircraft’s profile will be of the same basic form as this curve; which is steeper initially and flatter at the end. If predicted altitude at distances A and B (or any other) from present aircraft position can not be extracted because a type specific curve is not available, one might conclude that the curve is of academic interest only, but

there is a real-world compromise. For example, in a heavy Airbus A320, the QRH data tells us that it takes 60 minutes and 330 Nautical miles to drift from FL350 down to FL195. If we were to assume a linear relationship (which we know it is not), then this would equate to an average ROD of circa 270 fpm. We know that the initial rate will in fact be higher than this, as corroborated by the 300 fpm figure value referred to earlier and which was achieved in a simulator. Conversely, the final ROD will be far less than 270 fpm, indeed miniscule as we get closer to our stabilising altitude. Take advantage of simulator practice on your aircraft type in order to know the performance you can expect if the worst happens.

What implications does all of this have for your decision as to where to point the aircraft? Although you do not know how steep the curve illustrated in 19.5.4 is for your aircraft on the day, you should (based on the above) have an idea of what ROD to expect. Your proximity to terrain may dictate that you make an immediate decision based upon these. However you will in all probability also have time to see what descent rate your aircraft actually settles at before making/confirming your decision.

Ideally you have already made your decision before the failure occurs. Imagine an engine failure approaching the Alps with an MSA of 18,300 ft in the above example and you expect to be at FL195 after another 330 n.miles. Although this predicted level off is uncomfortably close to MSA, you might justify continuing if the failure occurs very close to Mont Blanc because you know that in reality you will be nearly 330nm past the high ground of the Alps before you actually reached that altitude. Conversely if Mont Blanc was 330nm ahead of you when your engine failed, would you really want to continue? (Ignoring for a moment the twin engine requirement to divert and land as soon as practicable, as discussed earlier).

On your aircraft type, would your decision to continue change if a second engine failure occurred? To that end, what is the likelihood of a second engine failure, given what is known about the first one? Was it clearly limited to that engine (e.g. fire, catastrophic failure or shut-down due to high vibration)? Could your remaining engine(s) be affected (say by a volcanic ash encounter, icing or a fuel contamination / starvation / leak)? Perhaps the engine ran down for no discernible reason (computerised engine control or electro-magnetic interference)? Each scenario may subtly influence your decision whether or not to continue across the high ground. You might simply conclude that you always prefer to divert to an airfield (if one is available) on your side of the high ground, unless you are very close to, or directly above the highest peak at the moment of failure, and the terrain beyond it is known to fall away rapidly thereafter.

Some operators formalise the decision making process into pre-defined ‘escape routes’ for high MSA sectors above a certain aircraft weight. In the event of any engine failure a pre-determined route is flown which varies according to which stretch of the mountainous area is being over flown when the engine fails. Clearly it is imperative that the pilots know precisely which segment they are in at all times, so that the correct ‘escape’ is flown. Such escape routes are not just for twin engine aircraft: Multi engine aircraft like the B747 have escape routes in case they should lose two engines when routing across, (for example), the Himalayas when heavy. On a multi engine aircraft the loss of one engine does not necessarily necessitate a diversion. On a twin engine aircraft you will be seeking to land as soon as is practicable / at the nearest suitable airfield. The important thing is that you DO have a plan and that your crew have one too. As Captain of a long haul aircraft you may well be asleep in your bunk when that engine fails, leaving the decision and initial actions in the hands of your capable co-pilots!

b. Standard Strategy (non Terrain-Critical: Normal descent to engine out Long Range Cruise altitude) If the stabilising altitude is above MSA for the remainder of the route, then it is safe to continue for terrain clearance purposes, although the aforementioned considerations may still dictate whether or not you actually choose to continue.

The Obstacle Strategy uses large tracts of upper airspace and makes increased demands on the crew due to the very low rate of descent. Your own diversion requirements, or those of ATC, may dictate a more expeditious descent rather than a drift down. Hence, if terrain is not critical, then starting with the obstacle strategy buys time to get an ATC descent clearance; but thereafter, a normal descent rate and speed at idle thrust can be flown to your desired altitude for diversion, or whilst continuing along your route.

You may still need (for terrain clearance) to descend to the same altitude as would have been achieved using the obstacle strategy. If so, remember that you will end up slightly lower, since you will arrived there sooner hence heavier than in the drift down case.

This stabilising altitude is a ‘maximum possible’ and initially requires MCT (later reducing with aircraft weight). Therefore in order to reduce strain on the engine (MSA, fuel and range permitting) it may be preferable to descend to a Long Range Cruise (LRC) altitude several thousands of feet lower than maximum. Therefore when / if you have no requirement to maintain the maximum stabilising altitude, descend to your chosen LRC altitude using normal descent speed and ROD, with idle thrust then resume level flight at LRC power setting and speed for the aircraft weight. Maintain these to the diversion or destination.

To conclude, it is essential to plan ahead when approaching high terrain such as the Alps, to establish the

WHAT/F scenario should an engine fail (see earlier text on MSAs), fly the Obstacle Strategy initially, then convert

to Standard Strategy when able (which may be immediately). Establish a stabilising altitude and the distance that will be covered whilst drifting down to it. Compare these with the worst MSA ahead of you and your distance from it. This allows a considered decision on whether to turn back before the mountain range is reached for a diversion

on the ‘approach side’ of the high ground, if the aircraft is expected to reach stabilising altitude below the MSA for the sector. If it is safe, carry-on, fly over the high ground, then divert for a landing beyond it.

Aneto range, Pyrenees Wikimedia Commons – GNU FDL photo by Avh