Very low payload threat?
 

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...

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. :}

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.

FCeng84

with the appropriate FADEC standard installed and certified you can reduce T/O thrust by up to 40%

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 ...

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.

Ok then find yourself needing to change thrust setting after you've pushed. You gonna draw another loadsheet at the hold grandad?

Good. 25 years later after this: https://aviation-safety.net/database...?id=19940630-0

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

I would answer the question...

Indubitably. Try and pose something challenging :rolleyes:

Your alternative to a smooth landing is floating hundreds of feet past the TDZ?

No, instead of being deliberately schismatic, reread what I typed. :ugh:

The biggest threat with those low payloads is you aren't going to make money.

If you float hundreds of feet past the TDZ there's something wrong with your technique.

I certainly do having been doing so since 1979, thank you.

Still it would be nice if you answered my question instead of avoiding it.

kindof SOP for the 388

Congratulations. Many don't which is why I highlighted it.

Now, any more silly assumptions from you?

An excellent and valuable contribution to the thread. The OP will be impressed by you if nobody else is.

No, the fact that you directly quoted me lead me to believe you were addressing me.

Used to fly embraer 190 series - If I remember rightly there was a restriction of 2 or 2.2 tonnes of fuel that had to be kept unburned in the tanks (ie treated as ballast).

172_driver - while I have not been involved in the 737 design I am familiar enough with the system to take a run at your questions.

The 737 variable pitch feel on a specific derivative is a function of airspeed and stabilizer position on all 737 derivatives. Between 737 derivatives the dependencies on speed and stab have been modified as needed to address handling characteristics of each derivative. On the more recent derivatives (starting with NG I believe) the variable feel has been updated to include: (1) features to increase forces at high AOA to provide better approach to stall awareness, and (2) an upper limit on feel setting for takeoff to avoid higher than desired takeoff rotation forces when heavy and taking off at higher speeds.

The variation in feel with stabilizer position provides compensation for the variation in pitch response to elevator with changes in CG. The stab tends to trim more airplane nose up at fwd CG where airplane maneuvering requires larger elevator displacements. Correspondingly the variable pitch feel is softer there. As CG moves aft, the stabilizer trims more and more airplane nose down and the variable feel stiffens.

The 737 variable feel does not have knowledge of airplane weight and thus does not vary on a given derivative with weight provided the trim stabilizer position is constant. (See paragraph above.) There is, however, some variation in feel schedule between derivatives in an effort to make the feel similar across the fleet.

When comparing airplane response between derivatives of a given model it is interesting to consider the geometric, inertia, and aerodynamic differences between a shorter / lighter derivative and a longer / heavier one. As length / weight is increased pitch inertia will go up, but so will the tail moment arm (distance from CG to elevator). Having a longer moment arm will partially compensate for the increased inertia such that similar handling qualities are usually achievable with the same sized elevator and the same gearing from column to elevator. It is to be expected that a light weight 737-600 will have snappier response than a heavy 737-800. I would be interested in your comments on how the variation in response on a 737-600 between when it is heavy shortly after a MTOW departure and when it is lighter during approach at the end of that flight compares with the range of characteristics you find on a 737-800 over its weight range. Along the same lines, how do you find the difference in response between a 737-600 and a 737-800 if both depart at their respective MTOWs?

FCeng84

Well I was being sarcastic...

About 35 years ago, I was flying from Denver to Seattle - a huge snowstorm had hit Denver and everything was massively fouled up. My incoming flight arrived several hours late, which caused me to miss my connection, so I was on multiple waiting lists trying to get on a flight to Seattle. Finally hours later - around midnight - I found a United flight that would take me. Amazingly, with thousands of people stranded and huge waiting lists to get to Seattle, the DC-10 was nearly empty - maybe a dozen people in coach. :confused:
As we were preparing to take off, the pilot came on and said he needed everyone to move to the front of the aircraft (including the flight attendants) to get the CG right for takeoff. Up and away we could move anywhere we wanted so long as we stayed out of first class...

For the CoG: The A321 is quite nose heavy, and depending on the your weight and balance policies (operational limits) it might become an issue. A320 and A319 not a concern.

I remember in the 90s (around 96') we had an additional restriction on Minimum flight weight. If this weight could not be acheived by payload, then we needed to use fuel to bring the weight up to this limit. This additional fuel would be classed as unusable fuel and fuel calculations for the flight would not take this fuel into account.

This was when I worked with Fedex and the aircraft was an A310F. I understood this was due to the less desirable handling of a light aircraft.

Anilv

I recall a flight many years ago (I believe on a 737) that was only about half full. Seats were not assigned, but we as SLF found the first 15 rows or so taped off as not available. The cabin crew when asked indicated that the reason for having us all in the back half was balance management. I also recall a ride in a small private airplane with a friend where his walk around included manually testing the tail-dragger weight by trying to lift the aft fuselage off the ground. Finding it too heavy he re-positioned a rather large toolbox from the aft end of the cabin to under my feet at seat 0B to get the balance right. Good lessons that there cannot be any shortcuts when it comes to maintaining weight/CG within the design limits!

It was Boeing that came up with that idea!! If interested I could send you the details how we did our loadsheet with adjusted weights.

There's a post of mine somewhere here about a onetime BMI flight from Inverness to Heathrow, normally a domestic A319, but instead one of their mid-haul A320s had been sent. These had a huge business class cabin for an A320, but being a domestic flight it was only eonomy pax. For W&B reasons however they could not be nicely spread across the rear cabin which had around 80 Y seats. There were around 50 pax booked so all were squished 6-abreast in the first 8 rows of Y, with the remaining rows behind, and all the business seats, empty.