Confusions on Alternate Flaps & QRH "Both engines failure"

[B738HT]

Question:
1. What if we decide to extend TL flaps with standby pump, and we put the alternate flap trigger on "ARM", does this disconnect the PTU? If YES, is that reversible？

2. For "Both engines failure" checklist , why we just put the thrust levers at present position( I mean there is no memorized items for thrust lever), but if we try to start the engine up later with X-bleed or APU ,we must close the thrust lever.

[FlyingStone]

I imagine you are talking about 737.

1. TE flaps are extended electrically in alternate configuration, LE flaps & slats hydraulically using standby system. As far as I know, PTU is only used for autoslats (but even for this you need fluid in system B), so when you have complete loss of hydraulic system B, you cannot retract LE devices after they are extended.

2. In dual engine failure situation, you need the thrust NOW. With an in-flight start of a single engine, priority is to monitor that engine starts normally (EGT, vibration, etc.) and to shut it down if abnormal start occurs.

[RAT 5]

B738HT:
1. It depends on why you are using the TE ALTN system. If it is for a flap problem then I assume HYD B is available. You ARM the ALTN flap system and then momentarily select the electric flap switch down. The LED system goes directly to FULL EXT position. You have disconnected the HYD B from all the flaps and connected LED to SBY & TE to electric. The LED's cannot be retracted via the ALTN system. If you have HYD B available, and close the ALTN flap ARM switch you can retract the LED's depending on where you position the flap lever. Of course, if you move the flap lever it will also command the TE flaps to move.

2. The LOSS of THRUST on Both Engines is a very different scenario to an Engine Inflight Start. With total loss you want a recovery ASAP so speed (windmill) and Ignitors are your priority. Engine & FADEC buffs will know more, but I suspect that a high speed windmill start might be more successful with TL commanding a little more fuel than idle. I'm not an engineer and welcome the real reason.

[tdracer]

RAT 5 has it pretty close for an all engine power loss. I'm not very familiar with the specifics of the 737NG, but with FADEC the throttle position is pretty much meaningless during a start - the FADEC is going to do the same start scheduling if the T/L is at TO as it will at idle. The important thing is to cycle the fuel switch so the FADEC is thinking "start", not "relight" (again, not familiar with the NG but on GE90 and GEnx the FADEC has a lot more flexibility when in start mode), and momentarily cycling the fuel OFF then back ON will often clear a stall or surge.
However shutting down an engine from power is hard on the engine - rubs compressor seals and such and can cause permanent performance loss. When we shutdowns and restarts during flight testing, we normally cool the engine at idle for at least 3 minutes prior to fuel off, then after it's running again let it warm for at least 3 minutes prior to advancing the throttle to avoid unnecessary deterioration.
An all engine power loss is assumed to be an emergency, so we're not worried about potential economic damage to the engine but rather in making thrust again as soon as possible. If the throttle is off-idle during the restart, as soon as the engine reaches idle it'll keep accelerating to the commanded thrust level.

[B738HT]

The reason why I'm confusing is I retracted the LED during go-around with the loss of system B pressure on the simulator. I read the AMM about PTU, it says when alternate flap master switch on ARM AND alternate flap position switch DOWN, it will inhibit PTU working. For single engine(starboard failure) going around, we lost B system EDP pressure, TE flaps in 15 and on air, all conditions satisfy the PTU operation, but the QRH says as long as we extend LED, WE CAN NOT retract it.

[B738HT]

Let's follow the QRH, after we finish the memorized items, neither engine starts, then we execute windmill start or APU assistant start depending on N2, but both procedures need the thrust lever idle. My question is not just for single engine start.

[Avenger]

When using alternate flap extension the LE flaps and slats are driven to the full extended position using power from the standby hydraulic system. In this case the ALTERNATE FLAPS master switch energizes the standby pump and the ALTERNATE FLAPS position switch, held in the down position momentarily, fully extends the LE devices.

Note: The LE devices cannot be retracted by the standby hydraulic system.

In a nutshell, the PTU becomes isolated along with the hydraulic system to prevent hydraulic lock, as mentioned before, the PTU is only for autoslat operation

I'm guessing in the sim you had the APU on the bus and the load shedding of the electrical hydraulic pumps didn't occur

[Avenger]

Back to the Thrust lever position: First sentence in QRH is objective to START AT LEAST ONE ENGINE.

In normal operation the FADEC will control the fuel flow according to the TL position.

http://www.boeing.com/commercial/aeromagazine/aero_02/textonly/sy01txt.htm

RPM Hang-up During airstart

During airstarts at high altitudes and low airspeeds, rpm hang-up may be experienced after obtaining initial light-off and acceleration. The hang-up is characterized as a stabilized engine speed lower than idle, with no response to throttle advancement and may be the result of either too rich or too lean fuel scheduling, low airspeed or excessive altitude.

The inflight start envelope defines the region where windmill starts were demonstrated during certification. It should be noted that this envelope does not define the only areas where a windmill start may be successful. The Loss Of Thrust On Both Engines NNC is written to ensure that Flight Crew take advantage of the high RPM at engine failure regardless of altitude or airspeed. Initiate the memory portion of the LOSS OF THRUST ON BOTH ENGINES NNC before attempting an APU start for the reasons identified above.