B737NG target speed deviation during cruise

[JetB737NG]

Dear all,

Sometimes I notice that the B737NG starts to deviate from the FMC target speed during cruise, and when the deviation becomes too big it adds thrust to correct for this. Although all settings in the FMC PERF INIT page are set-up correct and there are no significant temperature/wind deviations it tends to do this.

Could anyone of you, maybe a technician, come up with a reason for this (e.g. different FMC thrust/speed schedules).

Thank you for sharing your ideas and inputs.

[Tee Emm]

Is it possible the actual payload weight is different from the planned zero fuel weight input into the FMC? For the 737-200 it was discovered that 1200 kgs overweight from what was the entered ZFW will give a loss of some 10 knots in cruise from planned numbers.

[framer]

...... Temperature change.....The fix to this common problem is very simple, ask mum to turn off the heater.

[lederhosen]

For us no speaky 'strane lingo mate, perhaps Framer could translate his simple fix.

As regards the original question I find some of our aircraft are noticeably worse at holding selected speed in all regimes. As the FMS software and flight planning are otherwise consistent this makes me wonder if there could be other factors at work. Performance of older aircraft is known to deteriorate in areas such as fuel burn. It is not beyond the bounds of possibility that there could be an aircraft specific factor at work.

[Linktrained]

A possible remedy ( if cruel ) might be to turn on the "Seat Belt" sign !

Have a look at :

linktrained toilets

on Google. Then go to #395.

I'm sorry, this is the briefest way for me too redirect you. ( Historical note : A York was also an aeroplane, not unlike a 737 in some basic respects, I think.)

Sometimes it would feel as though a Cabin foot-ball match was being played, probably just a queue for the Loo(s).

[Capn Bloggs]

Surely the autothrottle must be stuffed?

[BOAC]

I'm pretty sure it is just a question of tolerance on the system - just like thermostats etc, they have an 'operating range' before they either 'switch on' or 'off', so the speed keeping will never be precise. As linktrained says,. pax movement will be one possible cause of changed speed. I have seen a 737 classic 'fail' to throttle back until it would have hit the max mach without manual intervention. There I suspect a knackered A/T system, and of course we all know how rubbish the 737 speed control in descent can be.

[lederhosen]

In previous times with 25 minute turnarounds, lower fuel price and resultant max speed cruise our aircraft would regularly overspeed into the clacker for example when exiting a jet stream. Our safety pilot and the fuel price have pretty much stopped this. But there are clearly situations the autothrottle cannot cope with.

As far as the seatbelts are concerned I find at least in the short term it makes things worse. It seems to have a pavlovian effect of encouraging people to get up and go to the toilet because they think otherwise they will be stuck seated for a while...not ideal when encountering unexpected turblence.

[ImbracableCrunk]

I've been wondering this myself. How much of this has to do with the how aggressive the FMC speed/autothrottle is when above the target versus below the target?

Why does it need to be any more involved than a simple wind change? The 737 wanders - get used to it.

[JetB737NG]

The question does not relate to the B737NG in general, but to specific aircrafts.

Some aircrafts in a fleet seem to maintain the speed better than others do, so this made me wonder if there could be another explanation for this (except the temperature/wind change).

[BOAC]

QUOTE from OP "there are no significant temperature/wind deviations?

[de facto]

I'm pretty sure it is just a question of tolerance on the system - just like thermostats etc, they have an 'operating range' before they either 'switch on' or 'off', so the speed keeping will never be precise

I agree and the AT response to be quite better when an IAS is selected rather than a Mnr...

[BOAC]

Spot on - I used to switch to IAS if necessary in the cruise and it was better- after all, the 'steps' are smaller.

[Linktrained]

A further item from the past:

Google

"fuel economist"flight 75

When fuel was cheaper, I HAD been taught that " The Cheapest Thing in Aviation is PETROL." And pressure pattern looked like the near future.

At Majendie's lecture on "Comet Navigation" to the Institute of Navigation, not then yet Royal, where Mach Cruising looked likely to be added to the Constant Power, Constant Speed and Long Range Cruise that we had studied, for Constellations 049, I think.

[Ian W]

The question does not relate to the B737NG in general, but to specific aircrafts.

Some aircrafts in a fleet seem to maintain the speed better than others do, so this made me wonder if there could be another explanation for this (except the temperature/wind change).

This sounds more like engines approaching major servicing as the turbine blade clearances become wider due to wear and thrust is lost and the FMC expecting more. The more engines are cycled up to high RPM and back the more wear there will be on the turbine blades. Every simple step-climb throttle up reduces the time between major maintenance.

[BARKINGMAD]

Once upon a time aircraft were placed "on the step", a speed at which the Mach/IAS seemed to be stable and well known by old codgers and F/Es, some of whom were still around in BAs Classic 747s.

If I recall correctly, the CFP cost index Mach for them was .84, but the experienced aviators (not bean-counters) would demonstrate that .86 was a much more stable cruise Mach # and the resulting fuel consumption increase was negligible compared to CFP, with less thrust lever "hunting".

Has the FMC cost index obsession deleted this knowledge from our database as we drive around controlled by desk pilots? :confused:

[paperdragon]

@BARKINGMAD Yes sadly you are all to correct...:D

[Linktrained]

On #14 I mentioned a Google reference to "Fuel Economist"

The Flight article (from 1975) was entitled "Precision Precedes Profits" ( which sounds a little over the top !). In the article it was stated that flying more economically increased flight times by 1 minute 23 seconds for each hour of flight for one fleet. (IE. not really noticeable in scheduling)

Yesterday Ryanair said that they were ordering their B737NG fleet to reduce their cruising speed to "save 15% of their fuel cost", thus increasing flight times by 2 minutes per hour.

Pprune has a wide readership. Congratulations !

Or was this just a coincidence ?

[Linktrained]

Possibly a further factor in your speed variations could be that you are flying when and where there may be standing waves.

The record altitude for a glider was 50,699 ft. by Steve Fossett over or near to the Andes in 2006. A powered aircraft would tend to climb (or descend) in these circumstances. With the cruising level held by the A/P, this would tend to vary the speed, or the power needed to hold a set speed.

I think "secondary" standing waves may be found on occasion, some distance downwind from the "primary" one.

[misd-agin]

Sometimes I notice that the B737NG starts to deviate from the FMC target speed during cruise, and when the deviation becomes too big it adds thrust to correct for this.

Happens to any airplane if you're trying to maintain a constant speed vs. a constant power setting(SEL aircraft). Hand flying, autothrottles, it doesn't matter. Speed variance is eventually adjusted when the human says "enough already". With a/t's it's the human that wrote the software code that decides when it changes the power.

Yes, the 737 wanders more than a lot of other a/c.