The last to know

Air Safety Week, Nov 1, 2004

Officials with the Transportation Safety Board (TSB) of Canada issued an advisory Oct. 20 to Transport Canada (TC), that country's regulatory body, on the proper weighing of cargoes. The action suggests investigators believe the MK Airlines B747 freighter that crashed on takeoff at Halifax Oct. 14 may have been overloaded (see ASW, Oct. 18). Eyewitness accounts of suddenly hearing a lot of power being applied midway through the takeoff adds to the scenario. That may have been the point of dawning realization by the flight crew - where they became the "last to know" they were taking off weighing more than calculated. Herewith, text from the TSB advisory, followed by a potential solution to what is reported as a chronic problem in the air cargo industry of underestimated cargo weights. First, the TSB:

TSB Aviation Safety Advisory, Cargo Handling - Verification of Cargo Weights (extracts):

"During its stop at Halifax, the [accident] aircraft was refueled and received a cargo load of seafood, which had been built-up into pallets by a local freight forwarder. The load was then delivered to a local freight services provider. The provider does not have the equipment necessary to weigh the builtup pallets at Halifax. Consequently, the provider used the average weights of individual boxes, which had been weighed by the packer, and the average weights of the lashing and loading equipment to derive the built-up weight.

"The crew received a pallet manifest, which indicated built-up weights, even though the load had never been weighed as a built-up unit. Equipment capable of weighing the built-up load is available at the airport, and it was the understanding of the operator that the freight services provider would conduct weighing of the built-up loads owned by another company. This equipment was not used ... and the weight was not verified prior to the accident flight.

"TC personnel ... have advised that ... there are no CARS [Canadian Aviation Regulations] which are directly applicable. Because there are no related regulations in place, regulatory oversight of freight services providers is minimal.

"It is not known if the weight of the cargo uploaded at Halifax differed substantially from the weight presented to the crew of the accident flight; notwithstanding, the risks associated with unverified cargo weights are substantial."

Real time take-off performance monitoring (as suggested by ASW contributing editor John Sampson of the International Aviation Safety Association, with acknowledgement to the collective ruminations of the Pprune Pilots' Forum):

"Real-time acceleration analysis during each and every departure roll might just be the 21st Century way to solve the problem, even if airport weight bridges or other passive controls are added as a further means to assure load limits.

"Many factors can influence the ability of an airplane to lift off the runway in the time and distance available: weight, airfield density altitude, runway slope, humidity, thrust, tire friction, winds, turbulence, runway surface contamination, etc. Some are fixed, many are variable, yet all are potentially conclusive factors in a maximized load marginal operation. When calculating what's possible, some factors (such as wind component) won't be known exactly. Few will be controllable once the take-off roll begins. The choices mid-roll devolve to power, airfoils (quickly selecting more flap), and reject/reverse/braking.

"Departure performance is pre-calculated with an expected weight, thrust, slope, winds, temps, etc. The calculation, as done manually, creates a few numbers such as MTOW and V-speeds. The calculations also will determine whether a reduced power take-off is possible (with increased engine life). But the same information can just as easily create a computerized moment-by-moment profile, expressed as speed versus distance, or acceleration vs. time, or roll distance vs. time from the application of thrust.

"Having derived this computed curve of expected performance at hand, it is the most natural thing in the world for a little bit of electronics to compute a moment-by-moment 'actual performance' during the take-off roll from groundspeed, GPS, wheel spin-up, acceleration forces, or all of the above. By continuously comparing the projected performance with the observed values, the resulting real-time performance quotient would provide a validated comparison of expected versus actual progress in getting airborne within the runway available. Any significant departure from expected norms would be alerted in a timely manner, rather than, at present, suddenly confronting the pilot with a huge quandary - to firewall it and go, or to try and stop. Such snap decisions by their nature have a 50 percent probability of being wrong. Actually, in a misconfigured marginal take-off, the calculated last point of abort will also be in error, so any such decision may have a 100 percent chance of being wrong (cannot get airborne within runway remaining but also cannot stop within the abort distance available).