Approach Speed vs Command Speed (737 NG)

[Oakape]

I'm sorry guys, but you 'bush lawyers' are half the reason there is so much confusion with what is required & what is not!

It was
"The command speed bug is set to the target speed you will be flying on the approach for both manual flying & with the autopilot engaged. So you set the speed you want to maintain - in your 1st example, 138kts"
which contradicts the Boeing 'advice' and indeed highlights the dilemma that advice is known to cause.

seems to be the problem here but frankly, it's not that difficult! And what exactly is it about my statement that contradicts the Boeing 'advice'?

Let me put it as simply as I can -

*The 'target approach speed' for autothrottle engaged is Vref +5.
*The 'target approach speed' for manual thrust is Vref + wind correction, with a minium correction of 5kts & a maximum of 20kts.
*The command speed bug is set to the 'target approach speed' on both autothrottle engaged approaches & for manual thrust approaches.
*If you disconnect the autothrottle during an approach, you should technically reset the command bug speed to Vref + wind additives.

'Target approach speed' is my term & I have used it to refer to the speed you have calculated that the approach should be flown at, either Vref +5 for autothrottle engaged approaches or Vref + wind additives for manual thrust approaches.

As for -

I NEVER bug Vref+5 with autothrottle in anything above a 'zephyr day' since the sometimes wild attempts by the a/t to maintain its 'programmed' margin are often too much to tolerate (for a sensitive soul like me).

This is outside the recommendations of the FCTM.

However, I do agree that the speed keeping ability of the autothrottle & sometimes large thrust lever movement can leave a lot to be desired. You may prefer to go outside the FCTM recommendations in order to deal with this issue, but I prefer to set the command speed bug as per the FCTM & simply over-ride the autothrottle manually when required.

[BOAC]

Let me put it as simply as I can

- yes, simpler and correcter

This is outside the recommendations of the FCTM.

- which bit is that? The FCTM actually says that UNLESS you are going to TOUCHDOWN with the A/T engaged, you should bug Vref+additives, not Vref+5 and 'tweak it' when you disconnect. (1.16) so

*If you disconnect the autothrottle during an approach, you should technically reset the command bug speed to Vref + wind additives.

is only correct for an unplanned A/T disengagement.

[Centaurus]

simply over-ride the autothrottle manually when required.

A Boeing 737 Memorandom from about 15 years ago advised against forcing the autothrottles against their clutch motors where the autothrottles are attempting to maintain the MCP speed. I am a bit vague but the figure given by Boeing was something like any more than 6 lbs manual pressure against the throttle levers while the autothrottle system is trying to do its thing, is likely to cause eventual damage to the clutch motors.

This would suggest that if you are unhappy with the performance the autothrottles are trying to give you in certain weather conditions, then it is better to completely disengage the autothrottle system and use manual throttle control, rather than "fight" the automatic system? Seems logical to me, anyway

[ImbracableCrunk]

I agree with BOAC in regards to the REF+5 and A/T: that is only for autolands. If you're planning an autoland, bug REF+5. Otherwise, bug the REF plus winds, etc.

[BOAC]

I C - absolutely, but the imponderable with the 'Boeing' teaching was that if you bugged your 'chosen' Vapp with A/T engaged in gusty/windy conditions, the kit would probably be flying you faster than bug and when you do 'disconnect' you would be faced with a power reduction to achieve bugged speed, hence my choice of manual thrust and fly what I want.

Oh yes, as 'C' says, do NOT over-ride the A/Throttle except in an emergency. Like any 'automatics' - if they are not doing what you need, take them out, don't fight them. NOT good teaching!

[Centaurus]

I am a bit vague but the figure given by Boeing was something like any more than 6 lbs manual pressure against the throttle levers while the autothrottle system is trying to do its thing, is likely to cause eventual damage to the clutch motors.

That should read "1.5 lbs of force - not 6 lbs.

Another Boeing originated document here: Inadvertent Thrust lever Interference. The autothrottle system is designed to provide over-ride capability. This is acoomplished by a force sensitive switch in the autothrottle mechanism. During the takeoff prior to THR HLD, a pilot following the thrust lever motion with his hand can inadvertently apply sufficient force to de-clutch the autothrottle servo-motor drive. This may temporarily stop the thrust lever motion, possibly resulting in an undershoot or overshoot of target N1."

[in FACT is]

Oakape, just read back the FCTM word by word to understand it correctly,

on page 1.12 FCTM October 31,2008 :

When using autothrottle, position command speed to Vref+5 knots.
Sufficient wind and gust protection is available with autothrottle engaged because the autothrottle is designed to adjust thrust rapidly when the airspeed drops below command speed while reducing thrust slowly when the airspeed exceeds command speed. In turbulence, the result is average thrust is higher than necessary to maintain command speed. This result in an average speed exceeding command speed.

Just imagine if you can work like an autothrottle than you can set the command speed to Vref+5kts, am I correct..???

Vref = 1.3 Vstall at actual configuration and you add another 5 kts, you are still far from stalling the airplane, while you still aimming to stall the airplane at very low altitude (10 to 5 ft AGL)

I asumming, whatever the condition are as long you are landing and set the bug to Vref+5 (consider head wind correction only) and you fly the entire approach with your corrected speed (head wind) you are in safe side as the : "This result in an average speed exceeding command speed"

then you : "bled off this speed as the airplane approaches touchdown".

even the FMC Approach page give you and +5kts as default.

correct me for my low understanding and poor english, thank you

[Oakape]

in FACT is,

read the paragraph after the one you have just quoted. It begins with "If the autothrottle is disengaged, or is planned to be disengaged prior to landing........"

You need to read the entire section entitled 'Landing' in order to understand what Boeing recommends, not just a portion on the section.

BOAC & Centaurus seem to be more up to speed than me, so I will defer any further comment to them.

[nick14]

So by the above statement about pushing the thrust levers, its not wise when going from a stable 40% to takeoff thrust to give them a prod after pushing TOGA? .

Edited to say:

And how about for a GA, push TOGA and give it a full arm of thrust, does that cause any issues?

[BOAC]

Nick - ASSUMING you mean 737???, we have discussed the take-off bit before on PPrune, and there is a somewhat old Boeing Tech paper on this incorrect 'shove them up for TOGA' and the associated dangers (due to the way the A/T actually sets TOGA). As far as g/a is concerned, no harm that I can see, and in fact standard procedure with manual throttle?

[nick14]

Sorry yes,

737-800. I will have to have a look for the paper as it seems to be important. Our full arm of thrust for GA is for both manual thrust of course but also for A/T engauged with the argument that if you always do it you wont forget if you are manual. Any harm in that?

Thanks

[Tee Emm]

nd how about for a GA, push TOGA and give it a full arm of thrust, does that cause any issues?

There are two issues. If you are flying a manual throttle approach and decide to go-around the only reason you press the TOGA switch is to bring up the FD needles to their GA attitude. Most pilots should be able to perform a perfectly safe hand flown go-around without the urgent need for FD guidance. However, other pilots are so reliant on automatics that a GA without FD guidance is almost akin to a Mayday. The Turkish Airlines B737 crash at Amsterdam was inevitable when the crew seemed mesmerised by the fact the throttles did not appear to work automatically.
If the auto throttle system is operating during an instrument approach, then commonsense would assume you guard the throttles and when TOGA is pressed and instant opening of the throttles did not occur , you would quickly set them at the go-around setting.

[nick14]

Tee Emm,

Thanks for the reply. Obviously we should all be capable of flying raw data under any stage of flight thats the whole point of being a pilot being able to cope when the automatics cannot. My questions was directed more at the effects on the a/t of pushing when its trying to do its job. Our SOP for a GA is to push TOGA (to get the aircraft in GA mode FD's and all that jazz) and push the thrust levers forward towards GA thrust. The PM sets the GA thrust (or monitors the a/t setting) I was just wondering whether doing so with a/t engaged causes an overshoot/undershoot or wears out the a/t motors or something similar?

[BOAC]

Nick - certainly unwarranted 'fiddling' (ie pushing) during take-off is known to be potentially hazardous (see Boeing paper). I can see no problem with a similar over or undershoot of N1 in a g/a where performance is slightly less critical. As to any mechanical effects, it is the clutch that is being over-ridden, and I don't think that would cause a problem. I personally go with Tee-emm's last paragraph.

[737ngpilot]

anybody got a link to the Boeing memorandum/Bulletin that's being referred to here

[Tee Emm]

have to disagree there, oak - you should aim to be at Vref(+) at 50' but bleed off to something like Vref-5 at touchdown.

Agree wholeheartedly. There seems to be a fear of flying at Vref. This speed has already plenty of fat built into it by certification rules. With regard to bleeding off the half HW component additive the FCTM says "approaching touch down." This statement frequently leads to a varied interpretation of "approaching touch down" Some assume it means during the flare and float.

The clue is in Supplementary Procedures (Adverse Weather) Approach and Landing where it says if ice formations are observed on the airplane surfaces, add 10 knots to VREF. This ensures maneuvering capability. There is also a Note that says "To prevent increased landing distances due to high airspeed, bleed off airspeed in excess of VREF+5 knots+gust correction when below 200 feet AGL. Maintain the gust correction to touch-down"

This statement suggests that the Boeing definition of "approaching touch-down" is when below 200 feet AGL. It does not mean after you flare.
Being on VREF at the point of flare is perfectly safe.