Boeing "thrust hold"

[slamer.]

This is something I have pondered on for some time and in light of events surrounding Asiana 214 at KSFO yesterday this seems like a relevant time to ask.

I understand what happens when the is AP is disengaged in FL change (or other vertical modes depending on type) and auto-thrust goes "HOLD". And the subsequent requirements with FD use.

What I don't understand is. Regardless of the procedure, why would the manufacturer philosophically want "thrust Hold" at such a critical phase of flight, potentially without any reversion.

Or to put it in simpler terms .. what were the people who designed this mode thinking..?

The FCOM usually provides the answers. But in this case .. not.

PS

I have asked people in our ATO. And even had a quick searched here without success.

Thanks

[galaxy flyer]

Not necessarily germane, but Bombardier auto throttles, Honeywell or Collins, in FLCH, will either command idle thrust or climb thrust, depending on whether the FL in the selector is below or above the present altitude or flight level. FLCH can control speed ONLY with pitch, so those thrust selections make a certain amount if sense--idle if going down, climb if going up.

To the present case, if they were in FLC, the autothrottles might hav been commanded to idle and might never have changed to SPEED mode. I do watch the A/T FMA and make call-outs of its mode, just like I do for FD and AP modes.

Not Boeing pilot, so pure spculation.

[Cubbie]

Actually the Autothrottle is not "disconnected" in flch and it is not trully disconnected in Hold mode, in fact the autothrottle is more like in a kind of standby - its disengaged but still armed at all times, or at least it should be unless called for by checklist to switch it off. The hold mode is provided to allow pilot input over the thrust setting as required, ie to modify the descent rate. As per the Boeing fcom; Autothrottle operation is controlled from the MCP and the CDUs. The MCP provides mode and speed selection. The CDU allows FMC reference thrust limit selection. When a pitch mode is active, the FMC selects the autothrottle modes and target thrust values.

The autothrottle can be operated without using the flight director or the autopilot. In this condition, the autothrottle operates in either the THR REF, SPD, HOLD or IDLE modes.

When the autothrottle is used during a manual landing, thrust reduces to IDLE at 25 feet radio altitude when the flight director is off or the pitch mode is V/S, FPA, G/S, or any VNAV mode (VNAV SPD, VNAV PTH, or VNA V ALT). The autothrottle does not automatically retard if the pitch mode is TO/GA.

With the autothrottle armed, the autothrottle automatically activates if no autopilot or F/D is active or an autopilot or F/D is in VNAV XXX, ALT, V/S, or G/S, and: • speed less than an FMC calculated value for one second • thrust below reference thrust • airplane altitude above 100 feet RA on approach, or airplane barometric altitude 400 feet above airport on takeoff The autothrottle can support stall protection when armed and not activated. If speed decreases to near stick shaker activation, the autothrottle automatically activates in the appropriate mode (SPD or THR REF) and advances thrust to maintain minimum maneuvering speed (approximately the top of the amber band) or the speed set in the mode control speed window, whichever is greater. The EICAS message AIRSPEED LOW displays.

Note: When the pitch mode is FLCH or TOGA, or the airplane is below 400 feet above the airport on takeoff, or below 100 feet radio altitude on approach, the autothrottle will not automatically activate.

Note: During a descent in VNAV SPD, the autothrottle may activate in HOLD mode and will not support stall protection

The autothrottle system can be disconnected manually by pushing either autothrottle disconnect switch, except during conditions that cause the autothrottle to automatically activate. The autothrottle can also be disconnected manually by positioning both A/T ARM switches to OFF, or individually by positioning the left or right A/T ARM switch to OFF. Positioning one or both A/T ARM switches to OFF prevents activation of all autothrottle modes for the affected autothrottle.

Autothrottle disconnect occurs automatically: • if a fault in the active autothrottle mode is detected • when either reverse thrust lever is raised to reverse idle • if the thrust levers are overridden during a manual landing, after the autothrottle has begun to retard the thrust levers to idle • when both engines are shut down The EICAS caution message AUTOTHROTTLE DISC displays and an aural alert sounds when the autothrottle is manually or automatically disconnected. The EICAS caution message and aural alert are inhibited if the disconnect occurs because of reverse thrust.

[BBK]

Best advice I've heard regarding this issue is as follows: if you see "HOLD" annunciated then it's a hint that you may want to hold the thrust levers.

[AtoBsafely]

Slamer,

As I think you know, in FLTCH the airspeed is being controlled by the elevator and the autothrottle will going into hold which is exceeding useful in allowing the pilot to adjust the ROD by repositioning the throttles. There is only a problem if the autopilot is disengaged and the pilot doesn't follow the FD pitch command.

It sounds like you are arguing that a stickshaker is not sufficient to prevent a pilot from stalling an aircraft. That may be true in this accident in SFO and in the DH8D in BUF, but that is a separate matter.

The fact that it is possible to stall the aircraft by misusing flight modes like FLTCH and VS is IMO not a shortcoming of the design as much as a condemnation of the training and licensing system that put that human in the pilot seat.