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Very low payload threat?

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Very low payload threat?

Old 27th Jan 2019, 12:26
  #21 (permalink)  
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Originally Posted by compressor stall View Post
No, instead of being deliberately schismatic, reread what I typed.
If you float hundreds of feet past the TDZ there's something wrong with your technique.

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Old 27th Jan 2019, 13:52
  #22 (permalink)  
 
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I consider this “very low payload threat” yet another indication of the lack of knowledge and over thinking of low hour and inexperienced pilots. No problem with asking the question, but why not pose it to your trainers or managers? Judging by the variety of contradictory responses, PPRuNe.org is once more (in my opinion), not to be regarded as an accurate source of knowledge.

Chesty M, I hope you would know what to do regarding the question I put, as should anyone regarding themselves a current line-pilot.
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Old 27th Jan 2019, 13:59
  #23 (permalink)  
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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.
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Old 27th Jan 2019, 15:45
  #24 (permalink)  
 
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Originally Posted by Chesty Morgan View Post
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.
I would have planned to have that contingency covered before I started the engines.

1979 ... a mere a sprog.
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Old 27th Jan 2019, 16:17
  #25 (permalink)  
 
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Can’t 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. You’ve got to consciously force it down.
kindof SOP for the 388
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Old 27th Jan 2019, 16:22
  #26 (permalink)  
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Originally Posted by Small cog View Post


I would have planned to have that contingency covered before I started the engines.

1979 ... a mere a sprog.
Congratulations. Many don't which is why I highlighted it.

Now, any more silly assumptions from you?
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Old 27th Jan 2019, 17:21
  #27 (permalink)  
 
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No, I think your remarks speak volumes.

BTW, your own vanity assumed that I posed a question and that it was directed at you. In fact it was rhetorical to the OP’s original title.
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Old 27th Jan 2019, 17:27
  #28 (permalink)  
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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.
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Old 28th Jan 2019, 12:15
  #29 (permalink)  
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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).
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Old 28th Jan 2019, 20:35
  #30 (permalink)  
 
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Originally Posted by 172_driver View Post

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?
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
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Old 29th Jan 2019, 07:54
  #31 (permalink)  
 
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Thanks for your inputs.

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?
During the summer charter season we do come close to MTOW in the -600. The variation in response between a heavy- and light -600 is somewhat what you predicted. The lighter, the more responsive. I particularly think the light -600 stands out as being very responsive, both in pitch and roll. The -800 is also faster in response the lighter it is - "you get more back for what you put in". All I say is based on my intuition of course, without bias for say CG-patterns on different flights.

Overall the handling differences aren't huge, but I particularly enjoy the -600 because of its flexibility. You can come in fast and it slows like you dropped an anchor. The approach speeds are slow so you can fly it tight and with ease predict where you'll end up. You can stop it on the runway before everyone else. But that is digressing from flight control responses and even further from the original thread title
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Old 29th Jan 2019, 15:01
  #32 (permalink)  
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Originally Posted by Chesty Morgan View Post
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.

Impressed? Why would I be impressed?

Apparently some believe all knowledge of aviation is known - there is nothing left to learn. Some believe they know everything about aviation - they have nothing to learn. Pilots who have those beliefs have been wrong, are wrong and will be proven wrong.

Those people donít impress me.
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Old 29th Jan 2019, 15:23
  #33 (permalink)  
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Well I was being sarcastic...
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Old 30th Jan 2019, 17:15
  #34 (permalink)  
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I knew that I was making another point.
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Old 30th Jan 2019, 18:02
  #35 (permalink)  
 
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CaptainMongo

Regarding your concerns about low payload. General talk -

Some aircraft types or sub-variants have restrictions on lowest permissible TOW/LW/ZFW. They must be adhered to.

Not all aircraft types have the facility to de-rate take-off thrust.

There may be reasons not to de-rate, eg runway length, climb performance, weather conditions.

How you manage those conflicting interests is between you (the Captain?), your Ops Manual requirements, FCOM, Manufactures official publications on the subject, and any Flight Manual requirements and restrictions.

If you do not think you have enough guidance as to how you can manage the situation then I suggest you ask your Flight Manager to provide advice in the Ops Manual. Any FM worth his salt should provide assistance to you.

If you are a Captain or a FO, and you are still unsure, then I suggest you seek advice from your Training Dept.
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Old 30th Jan 2019, 22:56
  #36 (permalink)  
 
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Originally Posted by Chesty Morgan View Post
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.
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.
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...

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Old 30th Jan 2019, 23:42
  #37 (permalink)  

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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.
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Old 31st Jan 2019, 06:49
  #38 (permalink)  
 
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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.

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Old 31st Jan 2019, 08:57
  #39 (permalink)  
 
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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
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.
With ACTs fwd CoG does become limiting on A319LR/ACJ.

Standard ops when full fuel with 4 ACTs (28T) with no pax to carry ~200kg of ballast to stay in balance, or if carrying pax to keep them all in the back until TOC.
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Old 31st Jan 2019, 19:27
  #40 (permalink)  
 
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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!
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