A/T mode confusion in TOGA 744

[Guest D17S]

"Tend to agree. I still like Cornish's theory that THR REF is not just a reference/power set to an EICAS thrust limit. THR REF always seemed like too big a name for something as simple as that."Making progress.

[cowpatz 243]

Q I tend to agree with you. The THR instead of THR REF made no sense at all to me on a derated takeoff. A normal derated TO has THR REF so why would it be any different if VNAV were not armed?Q what was the annunciated on the AT FMA as the aircraft captured and then maintained the MCP alt? Did it maintain THR REF until the flap limit less 5 and then revert to SPD? As the flap was retracted did it stay in speed or go back to THR REF. I was itching to give it a try out of LAX last night but I was being route checked and besides that sort of non-standard stuff is frowned upon on a revenue flight.Sam I dont agree with Cornish's theory. THR REF is simply setting whatever thrust limit is indicated on EICAS and supplied via the FMS. I have not seen anything else indicated in my flying experience. To me the name name implies perfectly what it's function is. Referenced thrust. CheersSteve

[cowpatz 243]

Hi Q,Today I had a simulator instructor specifically check this issue out for me and these are his findings and they seem right to me. VNAV not armed prior to TO.THR REF then HOLD at 65kts.Just after lift off the commanded speed is V2 to V2+25 depending on what speed you were at lift off etc etc. Thrust will be HOLD.Nothing happens at 400ft. AT's stay in HOLD.Nothing happens at 1500ft or whatever climb thrust setting was entered into the FMC. Commanded FD speed still V2 to V2 + 25.At the MCP altitude (was set at 2000 ft for the demo) pitch mode went to ALT and AT stayed in HOLD. The airspeed trend vector shot right up into the red bricks. When the airspeed got very close to the flap limiting speed less 5kts the AT's changed to THR REF and kept the airspeed at that setting. When the flaps were raised to the next setting the AT's stayed in THR REF and cranked the power up once again stopping at the next limiting flap speed less 5 kts.Once again I have still not seen it with my own eyes but I believe that that is what would happen.Do you think this because the FCC's crude flap speed limiting logic is dictating the AT mode and limits? I knopw that they have this capabilty in TOGA and FLCH.CheersSteve

[Guest D17S]

So in the big sim, THR REF is not just REFerened to an EICAS thrust limit. THR REF can also be REFerenced to a flap speed. Makes sense. It was just too big a name for a simple EICAS thrust limit REFerence.(. . . I'll bet it'll stop over at Vmo too)

[Qavion 19]

So the confusion still remains.... Two people intentionally simulating the same thing in different airline simulators and ending up with different results.Either there are real world aircraft/airline differences or the sims have different or faulty programming.As we can't check this out in the real world, we have to assume the sims are correct for our respective airlines.Anyway, many thanks for the feedback, Steve :-)Cheers.Q>

[cowpatz 243]

No Sam I dont think so. The thrust limit is still the one displayed on EICAS. It is only the logic for determining this limit that is decided by the FCC's if you follow.Q I would be very surprised if the sims are different. Surely this would be fundamental logic to all 747's even GE and RR which is the only difference between Cathay's Sim RR and ours which is GE.It depends how closely they were looking and when they were looking I guess. CheersSteve

[Guest D17S]

The EICAS thrust limit decreased to a new limit that provided a thrust level that precisely facilitated the thrust reduction necessary to control airspeed to the flap Vne (-5kts)? Wow! . . . but I seriously doubt that happened. If THR REF was annunciated in the thrust FMA, then the AT looked at the current flap Vne and used a speed-on-thrust scheme to control airspeed. I strongly imagine that is was not the EICAS thrust limit that decreased to facilitate this speed controlling thrust reduction. The description suggests that THR REF acted exactly like the thrust mode

[cowpatz 243]

Yes it is like all second hand info. Treat with caution. I will only know for sure when I next go in the sim myself and I am not sure when that will be.Maybe it is time to close this thread or carry it on under a new heading as it is getting a little hard to follow.RgdsSteve