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View Diary: How Airliners Work - Weight and Balance (160 comments)

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  •  Nicely done Major Kong. (10+ / 0-)

    I'd like to add something not often considered about weight and balance. It effects the performance of the aircraft in many less obvious ways than described in the diary. A heavy airplane requires more lift to fly. Increasing lift increases drag and a heavy airplane cannot reduces the altitude at which it can cruise. Each of these factors increases the fuel burn relative to a lighter aircraft of the same type.

    A very good example of this comes from my time in the early '90s when I flew the 747-400. Our company procedures called for all international baggage to be weighed. This is different from domestic service where the bags are counted and the total is multiplied by a FAA authorized average weight to come to a total baggage weight to add to cargo and passengers when figuring the Zero Fuel Weight Major Kong told us about. International passengers tend to have heavier bags so this procedure was intended to prevent problems with takeoff performance and excess fuel burn.

    My favorite destination of all time is Sydney, Australia. We operated LAX to Sydney four times a week. I began or ended every trip for three years with a SYD-LAX or LAX-SYD leg. One big operational problem we had is that it appeared to us that the staff at LAX was not weighing passenger bags as required. It really got our attention when we didn't rotate until well into the red center line lights (last 1500') on the longest runway and then could not coax the jet above 29,000'. Those are both top notch signs the jet is heavier than we were told.

    Every takeoff was at the maximum takeoff weight of 870,000# due to the 15+ hour flight time and maximized passenger/cargo load but even at warmer than standard temperatures the airplane should have been able to climb to 31,000'. Many crews stopped doing reduced thrust takeoffs in LAX to avoid any potential drama until our safety reports were finally taken seriously. I'm convinced the only thing that really convinced operations to look into the problem were the multiple diverts caused by excess fuel burn. I was involved in four in a six moth period. (Including Three hours at Noumea, New Caledonia, with their 1940 Ford fuel truck. ) Sometimes the only way to get an airline management's attention is to cost them money.

    An important take away from this is that there is a large margin of safety for weight and takeoff performance. We were almost certainly tens of thousands of pounds overweight but the airplane still got airborne safely. Loss of an engine or another performance losing technical problem would have been a far different story.

    My other point before I shut up is about CG. Aircraft manufacturers learned that an aft CG is great for reducing fuel burn. Many large jets have a fuel tank in their horizontal stabilizer in the tail. At first it was just one more place to put a fuel tank to get more range. Later, they filled that space during climb out to force an aft CG. The aft CG changes the aerodynamics of the aircraft by reducing the down force that the horizontal stabilizer needs to generate. The production of lift causes increased drag. Lessening the total drag equals lower fuel burn.

    The A330 tail holds 13,000# but almost never has more than 5,500# of fuel in the tail at takeoff. The load planners also arranged the cargo load to have an aft CG at takeoff to further take advantage of a cheap fuel saving trick. Losing the ability to transfer fuel aft results in a 1% increase in burn. An 8-9 hour flight which burns nearly 100,000# of fuel saves a modest 1000#. Not all that much until you add up all of the airplanes in the fleet.

    Time makes more converts than reason. Thomas Paine, Common Sense

    by VTCC73 on Tue May 07, 2013 at 07:23:28 PM PDT

    •  I flew KC-135s, like the Major, (10+ / 0-)

      In the olden days, we calculade our center of gravity and came up with an number, % of MAC, being Mean Aerodynamic Chord.  Chord is the length of a section of the wing, front to back.  The Mean Aerodynmaic Chord part comes in, because the wing is tapered, so the chord is different at each part of it's length, and the wing is swept, so the chord starts at a different point as you go out the wing, too.

      Many aircraft fly pretty good with the CG at about 25% of MAC, or about one quarter of the way back from the leading edge of the wing.  The farther forward it is, the more stable, the farther aft, the less stable, but as the CG moves aft from that 25% point, it becomes more fuel efficient, since it takes less downforce on the tail to fly level.  Our aft limit was about 37%, and forward limit about 16%.  

      We flew across the oceans using celestial navigation...shooting the angle of the sun or stars with a sextant.  Yes, really.  Obviously, the more stable the airplane was, the better shot the navigator could get.  And, we had a lot of fuel, all over the place, so we planned our fuel burn to move the CG forward when the navigator was going to be using the sextant, and moved it aft, as close to 37 as we could get, for long range cruise, and brought it back to about 25% for a nice landing.

      "We refuse to fight in a war started by men who refused to fight in a war." -freewayblogger

      by Bisbonian on Tue May 07, 2013 at 08:03:45 PM PDT

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      •  The magic in the modern jets is simply amazing. (11+ / 0-)

        I graduated from USAF UPT just a month over 39 years ago. The differences from those old days is a wonder I will never forget. The Tweet of 1974 had instrumentation that today would be considered marginally IFR capable. An attitude indicator that precessed in straight and level unaccelerated flight and was all black was just being phased out. Good luck with that POS in the heat of an unusual attitude recovery. One VOR, some had DME, one UHF radio that didn't always work if you flew through a rain shower, and the only precision approach was by ground based radar but with minimums of 100' and 1/4 mile. Flying at night was almost an emergency procedure just due to the horrible cockpit lighting. But at least you had a 350 mile range (<2 hours endurance, much less usually) with a good tail wind.

        The tanker was a bit better. Long range navigation by SAC decree was by navigator using the sextant you speak of. I was fortunate to have a fantastic navigator as a copilot. Vince was an INS in a flight suit. We always found ourselves exactly where he said we were. In June 1976 he dead reckoned his way from Torrejon AB, Spain to Platssburgh because there was no PINS (Palletized INS) available, the doppler went into memory as soon as we went feet wet, and the HF died just after our coast out report. A Navy C-130 crew passed our position reports for us. It was day time and we were at 28,000' under an overcast nearly the whole way. We were two mile off track and 2 minutes early at our coast in point. Vince did that every day. I still can't believe that I went most of the way around the world with those guys having almost no idea where I was.

        By the time I retired in November 2011 the jets had changed by leaps and bounds. The A330 knows its weigh and CG sitting on the ramp. Enter an erroneous figure for either and it yells at you and petulantly refuses to accept your entry, replacing it with its own. Navigation is so accurate that it is standard procedure to fly a one or two mile offset right of course to mitigate midair collision potential if air traffic control or a crew screws up their altitude or track assignment. The machine monitors parts you never knew you had and tells maintenance when they aren't working right. Oceanic control still considers the HF primary and mandatory for communication but we could talk to anybody in the world through satellite data link and/or satellite voice. Our clearances came via SAT data link (CPDLC). The clearances could even be uploaded to the flight management system with the touch of a button. ADS-B is on the horizon, close on the horizon, which will make nearly any part of the world able to be controlled as if in a radar environment. Aviation was an amazing place to live a working life.

        Yet, it is still about the basics like Major Kong describes so well. I'd wager he could still do a pretty decent job navigating RMI only. This stuff, like weight & balance, is never forgotten and nobody enters the brotherhood of aviators without mastering all of it.

        Time makes more converts than reason. Thomas Paine, Common Sense

        by VTCC73 on Tue May 07, 2013 at 10:02:17 PM PDT

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