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Very low payload threat?

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Very low payload threat?

Old 26th Jan 2019, 11:12
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Very low payload threat?

We know about threats associated with a high (heavy) payload. What are the threats associated with a very low payload? Ie A320 - ZFW 99,000lbs, TOGW 111,000, and LW 105,400.
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Old 26th Jan 2019, 11:21
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V1 will probably be VMCG. Faster off the ground as the max thrust reduction is 25%. High rate of climb giving TCAS alert if you dont use VS. Reducing from 160kts to a low Vapp in 4 miles.
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Old 26th Jan 2019, 12:22
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Canít speak for the 320, but the 319 is a bugger to land smooth when light (no pax, min fuel). It keeps floating and suddenly has enough. Youíve got to consciously force it down.

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Old 26th Jan 2019, 12:38
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One of the biggest threats operating at low weight is the narrowing CofG envelope. You can quite easily find yourself outside the allowable envelope unless you keep an eye on things. Particularly with multiple thrust limit lines.

And, I'd suggest, not landing smoothly isn't really a threat is it...
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Old 26th Jan 2019, 12:57
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Just adding a few bits.

Some of the Airbus fleet have long been approved for 40% thrust reduction.

V1 will be limited by VMCG(toga) whereas you would Flex a lot and possibly have an increased V2 schedule too. V1 choice from the V1 range has three obvious options in most of the software (including the one that prints the RTOW tables).
- V1 max
- V1 min
- V1 mean
- V1 range.

There is a minimum weight AFM limit on some of the Airbus configurations.


160 kt to 4DME in CONF 1 is a once in a lifetime idea.




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Old 26th Jan 2019, 18:45
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From the perspective of a flight control system design engineer without reference to any specific model, light weight / aft CG and heavy weight / fwd CG usually present the two extremes of airplane response characteristics that we must consider during design. Until recently augmented flight control laws for commercial transport airplanes have been designed without knowledge of the current weight and CG thus the closed loop response characteristics become a compromise between what is found at these two extremes of the weight / CG envelope. Heavy/fwd tends to present the most sluggish response and the highest required control forces as the airplane has its greatest inherent stability at this loading thus requiring more control input to maneuver. Light/aft the airplane is more responsive and has the lowest inherent stability thus can tend toward being overly sensitive.

Another aspect associated with being very light is increased response to turbulence. With lower inertia in all axes a given gust will bounce the airplane more. Add to this the higher sensitivity with respect to pilot controller inputs exciting structural modes and you usually have the greatest tendency for "bio-dynamic coupling" (BDC) at the lightest loadings. BDC occurs when the pilots grip on the controller provides feedback of accelerations that the pilot is experiencing on the flight deck into the system in such a manner that the unintended structural mode frequency inputs that the pilot is providing result in even higher flight deck accelerations. BDC can form a feedback loop that has sufficient gain to become unstable causing structural mode accelerations to grow. Some have viewed BDC as part of the greater PIO topic, but I find if helpful to differentiate between BDC as described here that occurs at structural mode frequencies and does not involve any cognitive, intentional pilot feedback and lower, frequency maneuver control PIO that involves the pilot intentionally closing the loop on one airplane response parameter or another thus creating a closed loop instability. Remedies for BDC and PIO are quite different so it is helpful to make this distinction.

Commercial transport control system design is moving more and more in the direction of sensing / estimating weight and CG for use when scheduling control law gains. This design degree of freedom allows the system to compensate for the differences in airplane response across the range of these two configuration parameters. One thing that we need to be careful about, however, is recognizing how augmentation may be masking the underlying open loop airplane characteristics. This becomes an issue when the weight and/or CG estimates that are being used for control system gain scheduling are incorrect leading to the wrong gains for the current flight condition. In addition, the robustness of weight and CG estimates may not be sufficient to preclude losing them due to detected failures thus having to revert to a backup set of gains or reversionary control laws that do not include scheduling based on weight and CG.

I fear I have digressed from the focus of this thread but appreciate the opportunity to share my perspective on these related topics.

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Old 26th Jan 2019, 19:29
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One of the biggest threats operating at low weight is the narrowing CofG envelope. You can quite easily find yourself outside the allowable envelope unless you keep an eye on things.
A load sheet for the flight would sort that one.

The prospect of flying an empty aeroplane must scare the willies out of you youngsters.
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Old 26th Jan 2019, 21:44
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Originally Posted by Capt Scribble View Post
... max thrust reduction is 25%...
with the appropriate FADEC standard installed and certified you can reduce T/O thrust by up to 40%
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Old 26th Jan 2019, 23:00
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My observation with very low weights is that the "pull the speeds out of the book without thinking" can bite the pilot very firmly if the sound stops near V1 .. especially if the operator norm is overspeed takeoffs.

Not a major problem to fix, but it does require some tailored practice in the box to see how the pussycat at normal weights and high overspeed takeoff schedules with a nice post-V1 failure becomes a raging tiger at min weight, min speed, max thrust, max aft CG, and a failure right on Vef and, of course, a low vis/ceiling setup. Mind you, those folk then never had a problem with a "normal" failure. It was interesting to see folks able to recover and then track out on the opposite LLZ after a few practice runs. Almost invariably, the first run was a loss of control situation which had to be frozen at an appropriate point in the sequence.

Granted the box may not model the point precisely .. but it does so to the extent that the pilot never forgets the exposure and the need for prompt action ...
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Old 26th Jan 2019, 23:48
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CS has a minimum take off weight defined by the deceleration rate in case of engine failure during capture mode.
Took a while to get my head round it but it's summarised thus:

Light weight gives very high climb rate and therefore it will enter ALT CAP very early. If an engine fails at that point the capture manoeuvre is not modified and so the aircraft starts bleeding speed trying to fly the curve. This can put you into underspeed pretty quickly if not addressed. Transport Canada didn't like it. So the fix was to restrict the minimum take off weight and therefore thrust to weigh ratio.
Lower climb rate means later ALT CAP and less time in the capture manoeuvre. Also slower deceleration after the failure which gives the crew more time to react.

A future modification to the AFCS software we are told will remove the restriction.
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Old 27th Jan 2019, 00:43
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Originally Posted by Small cog View Post


A load sheet for the flight would sort that one.

The prospect of flying an empty aeroplane must scare the willies out of you youngsters.
Ok then find yourself needing to change thrust setting after you've pushed. You gonna draw another loadsheet at the hold grandad?
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Old 27th Jan 2019, 05:08
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Originally Posted by FE Hoppy View Post

Light weight gives very high climb rate and therefore it will enter ALT CAP very early. If an engine fails at that point the capture manoeuvre is not modified and so the aircraft starts bleeding speed trying to fly the curve. This can put you into underspeed pretty quickly if not addressed. Transport Canada didn't like it. So the fix was to restrict the minimum take off weight and therefore thrust to weigh ratio.
Good. 25 years later after this: https://aviation-safety.net/database...?id=19940630-0



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Old 27th Jan 2019, 08:41
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Originally Posted by Chesty Morgan View Post
Ok then find yourself needing to change thrust setting after you've pushed. You gonna draw another loadsheet at the hold grandad?
What would you normally do?

Question; You have managed to get airborne, everything going normally, seat still dry, you reach your destination at an even lower weight, and on short final ATC ask you to go around. Could you cope?

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Old 27th Jan 2019, 08:43
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Originally Posted by Chesty Morgan View Post
And, I'd suggest, not landing smoothly isn't really a threat is it...
no but floating for hundreds of feet past your TDZ is.

And separately depending on where you are you might like to consider ATC. Coming in at 120kias or less might not be in their game plan, CONF3 gives a few more knots and less likely to have a go around behind.
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Old 27th Jan 2019, 08:58
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The only time we had a bit of a moment for an empty positioning flight on our BMA DC-9-15 was if the forward air stairs had been removed for MX. (They weighed 150 kgs)
We had to chuck in a couple of MG tyres in the forward hold to keep the CG in trim for the load sheet and/or have someone on the FD jump-seat for the ride.
Went like a rocket on take off
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Old 27th Jan 2019, 09:26
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Originally Posted by Small cog View Post


What would you normally do?

Question; You have managed to get airborne, everything going normally, seat still dry, you reach your destination at an even lower weight, and on short final ATC ask you to go around. Could you cope?

I would answer the question...

Indubitably. Try and pose something challenging


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Old 27th Jan 2019, 09:27
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Originally Posted by compressor stall View Post


no but floating for hundreds of feet past your TDZ is.

And separately depending on where you are you might like to consider ATC. Coming in at 120kias or less might not be in their game plan, CONF3 gives a few more knots and less likely to have a go around behind.
Your alternative to a smooth landing is floating hundreds of feet past the TDZ?
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Old 27th Jan 2019, 10:38
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No, instead of being deliberately schismatic, reread what I typed.
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Old 27th Jan 2019, 11:28
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The biggest threat with those low payloads is you aren't going to make money.
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Old 27th Jan 2019, 11:51
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From the perspective of a flight control system design engineer without reference to any specific model, light weight / aft CG and heavy weight / fwd CG usually present the two extremes of airplane response characteristics that we must consider during design. Until recently augmented flight control laws for commercial transport airplanes have been designed without knowledge of the current weight and CG thus the closed loop response characteristics become a compromise between what is found at these two extremes of the weight / CG envelope. Heavy/fwd tends to present the most sluggish response and the highest required control forces as the airplane has its greatest inherent stability at this loading thus requiring more control input to maneuver. Light/aft the airplane is more responsive and has the lowest inherent stability thus can tend toward being overly sensitive.
FCeng84,

Judging from your location one could think you're involved in Boeing aircraft design? One day I might find myself in a 45 tonnes 737-600 and the next in a 75 tonnes 737-800. There sure is differences in how the two planes handle, a light 737 is not far from a C172. In turbulence you're thrown about like a leaf and you have to resist the temptation of your bum, as it wants you to kick some rudder to keep her straight.

Are the flight control systems identical on all 737 models (-600 to -900) or are parts (in the feel system) modified to account for different stability and inertia properties? Does the elevator feel computer have any idea of the current weight of the aircraft?
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