*** Updated 18 Apr 2019 ***
Fortunately loss of thrust on both engines or double engine failure is an exceptionally rare occurrence. However it has happened and what has happened once will inevitably happen again.
In the event of total power loss QRH drills have been designed and written to enable a speedy and successful relight of one or both engines. Procedures and training have always assumed that this is achieved; culminating in, at worst, a single engine recovery.
What if a restart is not achieved or both engines are severely damaged, preventing a restart? To date little guidance or training has been available to cover this situation and crews have been left to their own devices with a need to draw on experience in dealing with total power loss gained and practiced on single engine light aircraft, perhaps many years earlier.
These notes are based on information derived from the Boeing 737-300 simulator and should be very representative of the aircraft. They are provided to give guidance on suggested techniques in the event of double engine failure and an unsuccessful restart.
It is assumed that all relevant non-normal drills are completed and that the APU is started and connected to the No. 2 Bus; this allows for normal gear lowering but flap selection on the alternate system. However, after gear extension consider taking APU electrical power off No.2 bus and connecting it to No.1 bus, this will then enable normal flap selection.
Note that Boeing policy regarding loss of thrust from both engines is that the only likely causes are fuel mismanagement, volcanic ash, or rain/hail ingestion. In these cases it is probable that an engine can be rapidly restarted if these procedures are performed expeditiously. Therefore, the object of the LOSS OF THRUST ON BOTH ENGINES procedure is to rapidly restart one or both engines and re-establish an electrical and cabin pressurization source. The flight crew should not wait until they are inside the In-flight Start Envelope. Accomplishing the procedure immediately allows the crew to take advantage of the existing RPM on the engines.
Immediately on recognition of loss of thrust on both engines the aircraft should be turned towards a suitable airfield for landing, this should be done at the same time as the appropriate non-normal drills are being actioned.
The only energy now available is speed and height; initially height should be maintained to allow the speed to decrease towards Minimum drag speed. Holding speeds in Vol. 3 are based on Min. drag speed and approximate to a minimum of 210kts at 44,000kgs increased by 5kts per 2000kgs weight increase. This speed must be maintained while the aircraft is maneuvered to a point close to a runway from which a power off landing can be made.
Minimum drag speed is the speed for the best Lift/Drag ratio and will give the maximum glide distance for a given height. Aircraft weight has no effect on distance flown and only a very small effect on rate of descent, at a weight of 47,000kgs the ROD clean is approximately 2000 fpm. A 180° turn will take 2000' and a 360° turn 4000'.
Two methods of recovery are available.
Both have their own advantages and disadvantages.
This recovery profile requires the aircraft to be positioned on the right hand side of the runway, abeam the threshold, heading in the direction of landing and just close enough to allow the Captain to look down on the touchdown zone.
The aircraft is then flown in a curve to the end of the downwind leg and from there a curving final approach path to touchdown. (See diagram)
Ideally aim to be at 4000ft minimum at the start of the procedure, at Minimum drag speed (Holding speed/210kts) with gear and flaps up. The aircraft will reach the end of the downwind at 2000ft lower than the starting height. Gear and flaps can be lowered at any time on the final part of the approach to suit the required descent path. But bear in mind that flap selection takes considerable time using the alternate system, unless power change over has been used (see the last para. of introduction).
This recovery procedure is infinitely variable and can accommodate a wide variation in starting heights simply by slackening or tightening the procedure. It does however require quite good visibility and cloud conditions.
Remember it is always better to err on the high side and have height in-hand, the spoilers remain very effective and can be used to lose height when assured of reaching the runway.
Over the runway if speed and/or height are excessive spoilers can be used to place the aircraft onto the ground. After landing extend the spoilers and thrust reversers for maximum drag and commence braking using one application until stopped.
For the straight-in total loss of power approach the aircraft must be positioned on the extended centerline of an ILS equipped runway at a minimum height (in 100's of feet) equal to 4 x the distance out, e.g. at 15nm, aim to be at 6000ft.
Establish on the localizer at Minimum drag speed (holding speed/210kts) with the gear and flaps up. Maintain the localizer and establish on the glide slope one dot high. Any speed increase during this phase should be accepted, once established speed variation will be minimal and should also be accepted.
When visual continue to fly the one dot high glide slope and, when assured of landing at least 1000ft. into the runway, and when below 500ft agl, lower the landing gear. Change the APU electrical power over to the No.1 bus and extend the flaps as far as possible in the remaining time, adjusting speed to the flap/speed schedule as they travel. When over the runway if speed and/or height are excessive spoilers can be used to place the aircraft onto the ground.
After landing extend the spoilers and thrust reversers for maximum drag and commence braking using one application until stopped.
Clearly this type of approach is suitable for weather conditions that preclude a visual procedure. The main disadvantage is that if the aircraft gets low on the approach slope there is no way of recovering the situation. Do not be tempted to reduce excess speed by early use of spoilers, landing gear or flap, it must be conserved until a landing is assured.
Thanks to Captain Mike Sykes for this article.