CL-604 Driftdown

Challengers

Larry sez:

Flying across a large body of water the idea of "feet wet" and "feet dry" rarely crosses our minds. We wouldn't leave Point A unless we knew we had enough gas to make Point B. But things can get dicey with two, three, or even four engines. If an engine quits, we might not be able to maintain altitude. And if we descend, we need to start worrying about many other things. Maximizing forward distance while minimizing loss of altitude becomes a top concern. You should know your driftdown procedures without reference to a checklist. In some airplanes, driftdown starts the second that engine quits.

Everything here is from the references shown below, with a few comments in an alternate color.

Last revision:

20170915

CL-604 Driftdown

With some aircraft their are added complications. Of course there are. Take, for example, any Challenger with a GE CF34 engine. You might need driftdown speed to make it to land. But if you were hoping on restarting an engine, driftdown speed might not be what you want.

A Case Study of "You Don't Know What You Don't Know"

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Photo: CF34 Interstage Static Seal (ISS) tolerances, from Engine 'Core Lock-up' Phenomenon, p. 8.
Click photo for a larger image

The 2004 crash of Pinnacle Airlines Flight 3701 brought to light a problem with General Electric CF34 engines known as "core lock." Modern jet engines are manufactured with very tight tolerances, as you can imagine. The advantage of very narrow clearances on turbine blades, for example, result in greater turbine efficiency. You get more thrust with less fuel, all good.

[Engine 'Core Lock-up' Phenomenon]

  • Core lock up has been a result of differences in thermal time constant between static & rotating hardware. This may result in interference of hardware in either axial and /or radial direction
  • Typically the most common region of interference has been in the hot section between rotating seals and static structure.
  • Bombardier has experienced, during various engine development history, lock ups due to turbine or compressor blade tip rubs, and air seals.
  • This phenomenon is a known potential condition that has been seen on many engines during their development and post certification, and is not limited to Bombardier aircrafts.
  • During initial aircraft certification engines are forced to 0% core speed and subsequently the aircraft is accelerated to confirm that the engines will not lock-up and can be windmill re-started.
  • Every engine potentially prone to ‘lock-up’, based on engine history, is verified during the aircraft FTP to not lock-up. Should a tight engine be discovered a ‘break-in’ procedure is executed.
  • Engine models with no history of lock-up are not specifically tested for this phenomenon, but every engine is presently windmill restarted near the upper left corner of the envelope.
  • Bombardier ensures that every ‘prone’ aircraft (e.g. CL300, CL604, CRJ200, DHC-8) is tested prior to delivery.
  • No CRJ200 or CL604 aircraft leaves BA production without both engines having passed the core locking determination procedure (robust process) and the windmill re-light procedure.
  • But what about engines that have been rebuilt by GE or third party vendors? I am told these engines do not bet the same testing.

So you might think this is a problem. The NTSB appeared to be satisfied with the answer from the aicraft and engine manufacturers:

[NTSB AAR-07/01, ¶1.18.2] As testimony during the Safety Board's June 2005 public hearing on the Pinnacle Airlines accident indicated, neither Bombardier nor GE considered core lock to be a safety-of-flight issue. The manufacturers claimed that engines that passed the screening procedure, with or without grind-in rework, would not core lock as long as the 240-knot airspeed was maintained.

Good enough, as far as dealing with the Pinnacle Airlines incident: keep at least 240 knots and you can avoid Core Lock. Now, back to driftdown procedures . . . Here's what Bombardier tells its crews flying the Challenger 604.

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Photo: Example CL604 AFM Driftdown Schedule, from CL604 Flight Planning and Cruise Control Manual, §05-03-2
Click photo for a larger image

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Photo: Example CL604 AFM Highspeed (0.72MI / 300 KIAS) Schedule, from CL604 Flight Planning and Cruise Control Manual, §05-02-1
Click photo for a larger image

[CL604 Flight Planning and Cruise Control Manual, §05-02-1

  • In the event of an engine failure during flight, a descent to a lower altitude must be expected, and the driftdown procedure should be selected. This requires maximum continuous thrust (MCT) on the remaining engine and a speed selected by the operator.
  • Different speeds may be used for the driftdown procedure depending on the operational condition. Driftdown data is provided for the following speed schedules:
    • AFM Driftdown Speed. This speed, referred to as the en route climb speed in the AFM, is close to the best lift-to-drag ratio and is used to maintain the highest possible flight level. This speed must be used if a terrain problem exists. It must be noted that the data presented is gross (actual) driftdown data, but terrain clearance must be verified using the net (degraded) performance data
    • High Speed Driftdown. Two speed schedules are provided for high speed driftdown:
      • 0.72 MI / 300 KIAS: This speed may be used to minimize the time to reach an airport when no terrain clearance problems exist. This speed also maximizes the distance that can be covered within a specific period of time after engine failure. It can be used to determine the maximum distance covered in 180 minutes after an engine failure, which may be needed to ensure an airplane operates in non-ETOPS conditions, under certain operating rules. The level-off altitude achieved with this speed schedule is significantly lower than that achieved with the AFM driftdown speed.
      • 0.72 MI / 260 KIAS: This speed may also be used when no terrain clearance problems exist. It results in a slightly longer time to reach an airport, or a shorter distance covered within a specific period of time, than the 0.72 MI / 300 KIAS schedule, but it results in better fuel mileage. The level-off altitude achieved is also greater than that achieved with the 0.72 MI / 300 KIAS schedule.
  • The performance data presented in this chapter are based on the use of maximum continuous thrust with air-conditioning on (one pack in high mode) and anti-ice off.
  • If after having reached the driftdown ceiling altitude and an obstacle problem exists, the driftdown procedure is maintained to fly an ascending cruise. If after driftdown and no obstacle problem exists, the subsequent cruise should be made using either the long range cruise (LRC) speed with power for level flight or at maximum cruise speed with MCT.

Is having a choice of three different driftdown speeds a problem? It could be. If you flight planning provider computed your driftdown speeds from the AFM chart, as they likely to do, your driftdown speeds are likely to be less than 240 knots. If you were thinking you could restart your engine at a lower altitude after driftdown, you might be surprised. Okay, you say, the airplane flies fine on one engine. Here's a personal story on this very aircraft type and engine.

I've made a lot of crossings in the 604 and never knew about this problem. I've done the big expanse between California and Hawaii many times. I've done the hop over the North Atlantic even more. Then there was more than a few between Alaska and Japan and further on to Hong Kong and Australia. But the one that this problem makes me think about was a time at 30°W. Everyone talks about being the worst place to have a problem. Well, there I was, when one of the engines had a vibration monitor go nuts. I retarded the throttle until the vibes came back to limits. Then a few minutes it would happen again. We ended up landing a few hours later with the engine near idle and they found out the front fan stage had completely delaminated. My plan was to keep reducing power on it until it quit. And after it quit, I would drift down at the AFM speed and restart it. Here we are, ten years later, I and find out that may not have worked.

I bring this up since Bombardier and your friendly instructor probably won't. You may find yourself in a similar situation in the future and it would be good to add this factor to your decision making.


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