B 737 Thrust Mode Display

[Guru8904]

Can somebody explain the absence of a DES mode indication in the TMD? Why is it that it indicates CRZ during the entire descent till capture of the G/S where it goes to G/A? I have gone through the FCOM but it doesn't say anything.

Thanks.

[B737900er]

Because there is no DES Thrust mode. When your descending the thrust is at Idle (most of the time). When you level off your "cruising".

[too_much]

It will stay in CRZ mode until 2000ft AGL then it will change to GA mode.

Think about the modes, they limit N1 settings, during descent you would not require an increased N1

[Guru8904]

It makes sense, thanks.

[JeroenC]

What's the purpose of CRZ at all? It's no limit, why not just CON?

[B737900er]

AT higher altitudes CLB, CRZ, CON are usually the same value.

[737aviator]

In my experience with our engines anyway, CON is always greater than CRZ even at a typical high altitude cruise.
At lower altitudes, CON is higher than CLB, but at higher altitudes, CLB thrust becomes equal to CON.

[RAT 5]

What's the purpose of CRZ at all? It's no limit, why not just CON?

I've also had the same question. It is a continuous limit so why not use just this indication. Max CRZ, Max Clb; they are not really MAX, CON is, so why all the differences? I'm interested in what is the MAX for the engine when I need it. TO is something else due to performance issues, but once that phase is over MAX = MAX n'est ce pas?

[No Fly Zone]

Simple Words...

Yup, I understood your reply. It would have been a bit easier, perhaps grasped in a single reading, if you knew the difference between 'your' and 'you're'. Pilots are pilots, not grammarians, I know. But please, do try to attend to the basics. Much grace is given to the non-native users of our horrible language, but I sense that you are a native - and you should know better.
Regards,

[STBYRUD]

Concerning the CRZ thrust rating: There was a bulletin a few years ago which warned about potentially poor A/T performance in mountain wave conditions which recommended setting the thrust limit to CON in cruise to have that little bit of extra power available in case you need it to counter a speed excursion. On the 777 it's a customer option to keep the thrust limit on CLB even drujng cruise, CRZ is still selectable but bizarrely only activates automatically during descent.

[None]

For the 700/800/900, there is also a statement to consider selecting ALT HLD in Mountain Wave activity and take the FMS out of the loop. Evidently that can improve A/T performance.

[B737900er]

Where can you find these Bulletins?

[STBYRUD]

In your company FCOM if they apply to your fleet?

[JeroenC]

ALT HOLD in mountain wave? Not CWS? What benefit does ALT HOLD have over other altitude modes?

[BARKINGMAD]

No Fly Zone. You are a pedantic (old) fart!

If not already, you will make a typical trainer.

Concentrate on the big picture and ignore the trivia.

NOT doing so kills pilots and their trusting customers, I hope you are not in a position so to do?

[737ngpilot]

The 737 autothrottle operates the same in both ALT HOLD and VNAV PATH by setting thrust to a value calculated to maintain speed in level flight and then slowly adjusting the target if not on speed. The autothrottle logic provides for more aggressive control of speed as the airplane approaches VMO or MMO.

In the Boeing wide body airplanes, VNAV does not respond as aggressively as the autopilot does in ALT HOLD

[ImbracableCrunk]

For the 700/800/900, there is also a statement to consider selecting ALT HLD in Mountain Wave activity and take the FMS out of the loop. Evidently that can improve A/T performance.

For the 700/800/900, there is also a statement to consider selecting ALT HLD in Mountain Wave activity and take the FMS out of the loop. Evidently that can improve A/T performance.

Those quotes seem to disagree. My company doesn't give us FCOMs, just their dumbed-down selections. Can someone post a quote?

[stilton]

Thrust limit remains on CLB during cruise on our 757 /67.



Useful sometimes.

[Kefuddle]

For the 700/800/900, there is also a statement to consider selecting ALT HLD in Mountain Wave activity and take the FMS out of the loop. Evidently that can improve A/T performance.

But the FMC calculates the approbate N1 regardless of A/P mode. I have not seen anything that even hints that the A/T response is different for MCP and FMC A/P modes.

There is a current bulletin concerning A/T response with certain software revisions such that if the thrust levers are idle in cruise for less than 60 seconds, response for the requirement for additional thrust can take too long. Amongst the recommendations is to not let the thrust reduce to less than 60% in the cruise and that CON maybe selected to help alleviate the lack of thrust at altitude.

The current Bulletins are in FCOM1.

[powershanks]

Nothing written to back me up, just a little experience talking:

I've definitely seen that 737 AT response is much quicker in ALT HLD once you knock the FMC off the loop. Perhaps it's just because you're cutting out an extra middleman in the chain of command.

I've been asked the CON/CRZ question by a couple of P2s. The best guess I could give them was reduction of engine wear, much like reduced thrust for takeoff and climb. It would be rough on the engines for the AT to keep increasing thrust up to CON for every random speed drop during turbulence. Thus a lower CRZ limit is programmed, which could consequently be jacked up to CON if the situation so demands.

Someone mentioned that TMD remains in CLB during cruise in his planes. That can happen on the 737s too, although it's an anomaly I've observed. When you transition between ALT HLD and VNAVPTH in cruise, I've observed the TMD change to CLB!!!