If you are cruising at a high altitude, as soon as the engine fails, you will notice a drop in aircraft speed. This is the first thing that needs to be addressed as a pilot. To address this a descent must be initiated to a lower altitude where the remaining engine can use the thicker air to create more thrust and maintain the speed at an acceptable level. This should not be delayed as the speed can decay at a quite a steep rate. Before initiating the descent, the engines must also be set from cruise thrust to Maximum continuous thrust (MCT).
Once the descent is initiated, the next thing to focus will be to attempt an engine re-light. That is, trying to get the failed engine started. If the attempt(s) fail, then the aircraft has to be flown at the lower One engine inoperative (OEI) ceiling and flown to the appropriate alternate. There are OEI performance charts in the manuals which the pilots can use to determine the fuel burn so that a proper decision can be made on where to divert or better continue to the destination.
When it comes to actual descent, we have three main strategies on how to perform it. They are (specific to Airbus aircraft, but it is quite similar in other aircraft as well:
- Standard strategy.
- Obstacle strategy.
- Fixed speed strategy.
STANDARD STRATEGY
In standard strategy, the aircraft must be descended at the stable engine windmill speed. This will help to restart the engines as at this speed, the engine rotors will turn at a speed sufficient to re-ignite the failed engine without the help of the starter motor. In modern aircraft, the level off OEI altitude can be seen in flight management system or it can also be accessed through the aircraft flight manual.
OBSTACLE STRATEGY
This is also called the drift-down procedure. The idea behind this strategy is to delay the descend to the OEI ceiling. This is beneficial when you have terrain on the route. The speed for the descent is the speed for best L/D ratio or the minimum drag speed, vmd.���. When the aircraft is put down on this speed, the rate of descent will be low and it will give time to the pilot to maneuver away from the terrain. The OEI ceiling when the aircraft is flown at vmd,���, the level of OEI ceiling will be higher than the normal OEI ceiling. However, once you are out of terrain area, the descent must be reverted back to standard strategy. The obstacle strategy is a make shift one.
FIXED SPEED STRATEGY
This is the strategy to adopt when you are in an ETOPS (Extended twin engine operations/ Extended operations) area. It is called fixed speed because the aircraft is descended at one fixed speed. When you are in an area affected by ETOPS, it is critical that you get to your pre-determined alternate in the certified ETOPS time. For example, if the aircraft has an ETOPS rating of 120 minutes, from the point the engine fails to the point when the aircraft reaches the alternate aerodrome nothing more than 120 minutes should pass. So, the speed for this strategy is devised by assuming an engine failure at the most critical point in the ETOPS region. At critical point it is assumed that the pilots will initiate the descend at the ‘fixed’ speed and continue flying on that speed to the alternate. Flying at this speed will guarantee that the aircraft reaches the alternate within the ETOPS certified time. So, this speed will vary from aircraft to aircraft and even from airline to airline depending on their operations.
Author – Anas Maaz ( Airline Pilot )