FMC speed vs IAS window speed
 

Hi All,

Not sure if this has been covered before (had a quick search to no avail), but was wondering if anyone knew a bit about the autothrottle on the 757/767.

When flying a fixed mach number (e.g. across the pond / at ATC request), there seems to be a general feeling that opening up the IAS window, setting it to Mach, then leaving that to control the speed does a better job than just changing the FMC CRZ SPD.

I wouldn't have thought it makes much of a difference - the A/T is being told to fly at 0.80 (say) by the window, or by the box - same thing.

Any thoughts?

Best wishes, and many thanks,

NW3

I don't know about the FMC on the B757/767 but on the MD-11 there is a difference between setting the speed on the FMC and setting it on the speed IAS window:

If you set the speed on the FMC and therefore fly this 'managed' speed, the autothrottle system works in a kind of 'smooth mode', trying to fly the FMC selected speed with as few thrust changes as possible, instead the aircraft is 'oscillating' around the selcted altitude (just a few feet in each direction) to maintain the FMC speed. Thrust changes are only applied by the system when really needed and then just very smoothly.

On the other hand, if you select the speed on the IAS window, the 'smooth mode' of the autothrottle system is disengaged, and the autothottle system is constantly adjusting thrust to maintain the selected speed.

Maybe the ATS on the B757/767 works in a similar way? Just an idea...

Regards,
DBate

Greetings,
The Speed you set in the MCP window speed becomes the TARGET SPEED, used by the FMC for the for the current Flight Phase, i.e., Climb, Cruise, or descent.:ok:

Very good question... I wouldn't have thought it would make a difference, but I've never flown long periods with the window open... maybe time to try it!

Only catch is make sure its the same as the figure in the FMC or the FMC predictions will be wrong!

Nothing in my books about it, but maybe someone with an engineering background might know more!

Aye, it's a good question and something that I've noticed myself. A number of times I've observed the speed get low (or high) without the speed window, so have then 'speed intervened' at the same speed as already selected then watched the thrust levers spring into action....

At first glance on the 757 with the window open the ATS will follow to match the speed commanded (no link with FMC) by moving the throttle.
With the window closed the FMC is in charge and issue a target thrust setting. The EEC follows the target by issuing a trim command to his respective HMU, hence no thrust movement unless out of his trim authority.
It does not explain why "it seems" having better perf with window open.

Actually that's an interesting point - in VNAV PTH and with the window closed, is the FMC demanding a given (reference) EPR/N1 or Speed?

NW3

In a Path descent, VNAV PATH changes to VNAV SPD during speed intervention. In all other phases, the pitch mode remains the same. In VNAV PATH, thrust controls speed; in VNAV SPD, pitch controls speed.

To MBCXHARM (post #5)

When you open the speed window, the thrust levers do not spring to life because the A/T would be more accurate with speed window open than with FMC in control.

In Cruise mode, the autothrottle is in a lazy mode, so as to minimize thrust changes. Thrust is kept constant as long as the speed, averaged over a certain time, is at the commanded value (this is a moving average). The instantaneous speed value may be off the target by quite a margin, without the A/T reacting.

When you change the target, either by changing the FMC target, or by opening the speed window, you will reset the moving average calculation, so any instantaneous deviation from the target speed will cause A/T reaction. Leave the speed window open for a long time and the same lazy behaviour may show up again.

Why is it that the speed either hangs above the commanded airspeed or below it ?? (via the window or FMC). It seems very rarely the A/T keeps up with what you want, particularly bothersome when you have to keep it within Mach .02 across the ocean. Could it be to do with the 1 g autopilot or am I just grasping at straws yet again ??

BTW does everyone adjust speed via the cost index or hard code the speed ? we do the former for cost savings. Not that I would know any better !! :)

If it's like the 777 the FMC target speed is just that.

There is an energy conservation logic built in to the AT system, which allows a window of speed before the thrust is adjusted, just in case it doesn't have to.

For the NAT we enter the cleared mach # in the FMC and then for the exit point pre-select ECON. That's easy.

Also on descent using FLCH instead of VNAV with the speed window open (or revising the descent speed in the FMC) will get quicker speed adjustments in my observations.

halas

Halas,

An interesting point to consider for your next crossing: after leaving Oceanic Airspace pilots must maintain their assigned Mach number in domestic airspace unless and until the appropriate ATC unit authorizes a change.

If the logic is the same as the B777 (and I suspect that it is), FMC speed variations in cruise and within 10 knots, is programmed to recover the target speed at the rate of 1 knot per 12 seconds (5 knots per minute). Larger speed excursions are recovered to within +/- 10 knots much more promptly. The logic behind this is to avoid thrust settings constantly "hunting" for the sake of passenger comfort.

With the MCP Speed window open, the logic is to maintain speed fluctuations promptly (e.g. during approach), and thrust response to maintain the speed is almost instantaneous.

Hope that that helps.

Best Regards,

Old Smokey

Ok Smokey I think I can answer this one.

The same question has been asked for years and with people opening the window and creating SOP's from hearsay, BA went to boeing a few years back and asked this exact question. The answer from Boeing was clear and straight forward. There is no difference what so ever between window speed accuracy and FMC speed accuracy. The guys opening the window are making no differnce at all.

Thanks Straight & Level.

Always get approval first. That is true. And on request we get it. Maybe thats because we are usually soooo far north.

halas