RB211 question

[arse]

On our 777s with RB211; on reaching cruise alt, as the power comes back, the EPR seems to drop below the cruise thrust setting, then slowly increase to the CDU indicated cruise figure. This happens even though the actual and commanded speeds are the same. The drop and increase is sufficient enough to make me wonder if there is a specific reason for it!

Thanks

[gas path]

Just a guess but, is it to ensure the a/c maintains and doesn't overshoot the commanded speed as there is a fair bit of residual thrust as the power reduces.

[kijangnim]

Greetings
What does the N1 do?

[ACMS]

Yes the CX RR powered 777's do the same. we don't know why, it just does.

No problems, It settles down to correct thrust and speed in about 2 mins.
Just keep and eye on it, as one should do anyway.

And the N1 does the same, it follows the EPR/Thrust lever movement back and then forward again, as it should.

Oh and technically the engine is a Trent 800, a derivative of the RB 211.

[Old Smokey]

During the asymptotic flare from Climb to Cruise mode, you may note that the Auto-Throttle annunciator remains in THR mode (There is no mixed THR and SPD mode). Naturally, as the climb rate decreases during this flare, the aircraft will overspeed slightly. Once in SPD mode, the A/T will reduce to apply deceleration to the correct speed, and then increase slightly to maintain it.

Regards,

Old Smokey

[Swedish Steve]

Oh and technically the engine is a Trent 800, a derivative of the RB 211.

But go and look at the data plate bolted on the engine, and its called an RB211 Trent.

[barit1]

While I don't pretend to know the unique dynamics of the RB, any gas turbine undergoes a rebalancing act during/after throttle changes. It's the result of different thermal time constants between rotor and stator hardware.

The rotors are massive and heat up/cool off slowly, so their critical diameters change slowly; the lighter stator cases/vanes/seals expand and contract more quickly, thus the clearances (which represent aero losses) are changing. Among other things, this transient can affect the fan-to-core thrust ratio split.

mere speculation: Maybe Rolls has added some compensating dynamics in their control to make the engine hold a constant thrust during this transient.

Further elucidation welcome!

[ACMS]

Old Smokey wrote

During the asymptotic flare from Climb to Cruise mode, you may note that the Auto-Throttle annunciator remains in THR mode (There is no mixed THR and SPD mode).

The Modes change from THR REF VNAV SPD to SPD VNAV PTH to level off the aircraft. The number of seconds this happens before the FMC Active FL naturally depends on the ROC but in my experience it always changes to SPD VNAV PTH.

I've never heard of "asymptotic flare" what is that exactly?

I've never seen the A/T stay in THR mode while capturing an ALT.

I'm not saying you're wrong but in nearly 7 years on the 777 I've never seen that.

[john_tullamarine]

OS can look to the Type specifics... but, for


asymptotic flare

the term shows an engineering science background... asymptotic (from asymptote = literally, not intersecting ie something in the nature of a tangent) is a math buzzword and is what the flight path is doing as it curves from the climb to the cruise, approaching the cruise path progressively more gradually. Flare is an upside version of what you do during the landing or takeoff .. hence a fancy way of saying that you gently bunt over from the climb to adopt the cruise ....

[SMOC]

Is it the same for VNAV and FLCH?

[ACMS]

ok thanks John.

The 777 goes from THR REF + VNAV SPD straight to SPD + VNAV PTH

Yes SMOC if you were in THR + FLCH SPD it will go to SPD + ALT ( basically the same )

After capture of the selected FL the Elevators are controlling the Altitude and the A/T the Speed.

Now the Bus is different, I believe they have a mode for "capture" and another for hoding it? ALT "star" is the capture I think??? Similar to LOC "star" on approach.