Weighty Matters – Miscalculation when loading can have unwelcome consequences (Aviation Week)

One of the wisest principles in aviation is to learn from the mistakes of others so that we don’t repeat the same. Part of the impetus to learn from others’ misfortunes is that aviation errors are often unforgiving.

On Feb. 2, 2005, a Bombardier Challenger crew helped fill a lesson plan when their CL-600 overran the end of Runway 6 while attempting to take off from Teterboro Airport (TEB) in New Jersey, then crashed through the airport fence, zoomed across a six-lane highway, clipping a car, and finally buried itself in a brick building on the other side. Miraculously, no one was killed. The NTSB investigation that followed revealed that the jet had been badly outside of its center of gravity (CG) envelope.

Unexpectedly excessive baggage; too many “plus” sized passengers; or unaccounted baggage in the nose section are problem areas endemic to business aviation

As a result, the Safety Board reiterated that a pilot in command is responsible for knowing an aircraft’s maximum allowable weight and CG limits before every flight. The PIC also has the responsibility to determine that the aircraft is loaded within those allowable limits.

Unfortunately, business aviation operations are particularly susceptible to overloading conditions. NASA’s Aviation Safety Reporting System (ASRS) database contains insightful accounts of flights involving improperly loaded aircraft, resulting from circumstances that could occur on any business aviation ramp.

The following is one such report by a Challenger 601 pilot and aptly illustrates the subtle avenues that led to a potentially dangerous situation.

“The takeoff numbers were computed based on information provided by the SFO [San Francisco International Airport] ATIS. Those numbers were based on a fuel load of 16,500 lb., no passengers and dry runway conditions. Trim was set to the high side of the takeoff position per the aircraft operations manual. Power was smoothly applied and positively brought up to 89.2%. . . . Aircraft acceleration was normal and without incident. The first officer called ‘rotate.’ The yoke was moved rearward. . . . With the yoke in the full aft travel position, the aircraft remained on the runway with no further response.”

Note that this occurred well past the takeoff decision speed. Thus, the flight crew was forced into an instant decision for which they were never trained.

“With ‘serious question of the capability of the aircraft to sustain flight,’ the takeoff was aborted. . . .The captain called the abort, applied max braking, deployed full reversers and the first officer deployed the ground spoilers. Stopping was accomplished without incident. . . . . The runway was cleared without incident. While the aircraft was parked, and not under motion at any time, the fuse plugs melted, resulting in flat tires. . . .”

Luckily the incident occurred on the long and wide runway at SFO. The aircraft was subsequently released by base maintenance for another flight. This is where the story gets interesting again.

“On the second takeoff attempt from SFO, the aircraft again required full travel to the rear stops of the elevator in order for the airplane to become airborne. Immediately after takeoff, full forward trim down, ‘stiff arm,’ was required to get the nose down. . . . This was the second incident of this type with this aircraft. The first occurred on a maximum weight departure from Washington National Airport bound for overseas.”

As long as CG is maintained within allowable limits , the airplane is designed to have adequate longitudinal stability and control.

The ASRS staff members decided this was such an interesting submittal that they called the pilots to learn more about the event. Accordingly, the report continues:

“The control problem was in the equipping of the aircraft. After the SFO incident, it was weighed in at 1,300 lb. over the basic operating weight. Equipment additions were built-up tires, jacks, a complete set of commercial aeronautical charts, etc. Galley dishes were found to be 200 lb. heavier than estimated. . . . . [NASA ASRS Report No. 367988, April 1997].

As the ASRS report illustrates, the problem did not result from an inability to perform a weight and balance. Rather, it alerts pilots to the need for monitoring the way in which an aircraft can be loaded outside its weight-and-balance limits.

As is well known, the CG, the point at which the aircraft’s total weight is concentrated, must be located within specific limits for safe flight. In airplanes, longitudinal stability is maintained by ensuring the CG is slightly ahead of the center of lift. This produces a nose-down force independent of airspeed and is balanced by a nose-up force, which results from a downward aerodynamic force on the horizontal tail surfaces and varies directly with airspeed.

As long as the CG is maintained within the allowable limits, the airplane is designed to have adequate longitudinal stability and control. If the CG is too far aft, it will be too near the center of lift and the airplane will be unstable and difficult to recover from a stall. By contrast, with the CG too far forward, the downward tail load will have to be increased to maintain level flight. This increased tail load has the same effect as carrying additional weight, requiring the airplane to fly at a higher angle of attack (AOA), thus increasing drag.

A serious problem caused by the CG being too far forward is the lack of sufficient elevator authority. At slow takeoff speeds, the elevator might not produce enough nose-up force to rotate and on landing there may not be enough elevator force to flare the airplane. This problem may not become apparent until a phase of flight at which the pilot’s operational options are limited to “bad” and “worse.”

The maximum permissible takeoff weight for an aircraft will vary for each individual flight based on many factors. The maximum structural weight won’t change from the flight to flight. Neither will the maximum zero fuel weight, which is another structural limitation based upon the maximum bending moment at the wing juncture with the fuselage. The maximum landing weight also doesn’t change, and that could possibly limit the amount of fuel and/or payload for a flight.

Refusing to load excessive baggage sometimes is an awkward matter to business aviation pilots.

However, the departure and arrival airports’ temperatures, altitudes, runway lengths and winds will limit an aircraft’s weight for takeoff. The max weight will also depend upon any obstruction-clearance climb requirements in a one-engine-inoperative (OEI) condition. Another limitation could include a requirement that a minimum amount of fuel remain in the tanks (possibly to ensure the fuel pumps remain submerged and not exposed to fuel vapor, which could ignite should a short circuit produce a spark).

Overloading an aircraft can have serious — sometimes disastrous — consequences. If weight is considerably heavier than in preflight calculations, planned takeoff speeds will be wrong. The aircraft will accelerate slower than predicted and consume more runway than anticipated. If an engine fails close to the takeoff decision speed, requiring an abort, the heavier aircraft will need more runway to decelerate and could roll through whatever is beyond the pavement’s end.

Alternately, if the overweight aircraft’s engine fails shortly after takeoff, the liftoff will have occurred farther down the runway than expected, thereby decreasing the distance available to clear any critical obstacles — that is, trees, towers, multi-level parking lots and such — in the airport environment. Secondly, the rotation speed will be in error on the “slow” side since it was calculated on a lighter weight, meaning there’s less stall margin in the climb-out. In this high-drag/low-energy state, the aircraft is on the back side of the power curve with a dangerous potential for further deterioration.

An overloaded aircraft has a reduced angle of climb and rate of climb. The former can be threatening during an OEI takeoff in which obstacles require assured climb performance. For example, on a typical day when launching off Runway 25 at Eagle County, Colorado, Regional Airport (EGE) on the Gypsum Five departure, the required climb gradient is 815 ft./nm to 9,200 ft. The delayed takeoff and climb leaves precious little altitude for skimming over that mountain saddle southwest of the airport in the event of engine failure. The danger is an overweight aircraft may not be able to outclimb the terrain.

The reduced climb rate can also be a problem at high altitude. If ATC directs a climb to some higher altitude but the extra weight taxes the aircraft’s performance, a pilot’s natural reaction would be to crank up the pitch angle or V-speed knob. However, doing so will put the aircraft further on the back side of the power curve. The heavier-than-predicted aircraft will also have a lower service ceiling than predicted. Upon level-off this can further decay airspeed, which, unabated, will result in a high-altitude stall.

A heavier aircraft will cruise at a slower speed, and the cruising range will be shortened. If the high-altitude buffet margins are based upon the loading information inserted into the FMS, the buffet margins indicated on the PFD will be in error. The actual margin between the low-speed and high-speed buffet will be less than indicated. This erroneous data could allow the aircraft to enter into the buffet, much to the pilots’ surprise. Additionally, the overweight condition could result in airspeed loss at high altitude when trying to top thunderstorms, penetrate a mountain wave, or encounter turbulence or some other unexpected atmospheric condition.

Know too that engines at altitude produce but a fraction of their power as compared to their output at sea level, and as described in “Training For High Mach Flight” (BCA, April 2015, page 46), the drag produced by a 30-deg. bank could put the aircraft on the back side of the power curve with no ability to accelerate. Turning at altitude in a heavier-than-calculated weight would exacerbate the problem. Maneuverability of a heavier aircraft is reduced, and that degradation is accented at high altitude.

Landing at heavier-than-predicted weights could also expose the aircraft to reduced safety margins. An approach speed calculated at the presumed lesser weight would place the aircraft closer to the stall. The heavier weight results in a longer landing roll and excessive loads on the structure, especially the landing gear.

A review of incident reports reveals problem areas endemic to business aviation that can result in over-loading or out-of-balance conditions: Unexpectedly excessive baggage; too many “plus” sized passengers; or unaccounted baggage in the nose section.

Refusing to load excessive baggage is an awkward matter, especially when the lead passenger insists other pilots accepted the same. But a professional pilot’s primary responsibility is to ensure safe passage, not accommodate overindulgence.

An aircraft’s CG must remain within acceptable limits through all configurations and phases of flight. CG shifts forward and then aft in flight as fuel is consumed first from the outboard and then inboard tanks. Fuel-use scheduling in swept-wing airplanes operation is especially critical.

In certain aircraft, something as innocuous as a pilot leaving the cockpit to grab a soft drink can put the plane perilously close to its CG limits. The CG range on some aircraft such as the Short C-23 Sherpa is extremely limited.

On March 3, 2001, a C-23B Sherpa belonging to the 171st Aviation Regiment of the Florida Army National Guard was carrying 18 construction workers from Hurlburt Field in Florida to Oceana Naval Air Station, Virginia. En route, one of the pilots left the flight deck to use the aft bathroom. His action so shifted the weight of an already unbalanced airplane that it became unstable to the point of uncontrollability while in an area of extreme turbulence. The violent g-force shifts then encountered rendered the crew and passengers incapacitated and unconscious and caused the breakup of the aircraft in flight near Unadilla, Georgia, killing all 21 persons on board.

The crash of a National Airlines Boeing 747 after takeoff from Bagram Air Base, Afghanistan on April 29, 2013, underscores the critical importance of properly securing all baggage and cargo. The NTSB determined that the five large military vehicles the aircraft was carrying had been inadequately secured and at least one rolled rearward, crippling key hydraulic systems and damaging the horizontal stabilizer components, which rendered the airplane uncontrollable. All seven crewmembers were killed in the crash.

Changes of fixed equipment may also have a major effect upon an aircraft’s weight. Any major alteration or repair work must be checked for conformity with FAA-approved data and signed off by an aircraft mechanic with Inspection Authorization or by an authorized agent of an FAA-approved repair station.

A repair station record or FAA Form 337 must detail the changes made or work completed. Any item added or removed from the equipment list alters the aircraft’s weight-and-balance record, which must be altered accordingly. A dated and signed revision to the weight-and-balance record must be kept with the maintenance records, and the airplane’s new empty weight and empty weight arm or moment index must be entered in the POH/AFM. Also, the new information needs to be entered into the databases used by dispatchers for computing weight and balance, as well as into the FMS of the aircraft.

If the alteration or repair adversely causes the aircraft’s new empty weight and CG to fall outside of its limit, permanent ballast — typically blocks of lead painted red, attached to structure and marked “Permanent Ballast — Do Not Remove” — can be installed.

Each cargo hold has a structural floor loading limit based on cargo weight and the area over which it is distributed. To determine the maximum weight of a loaded cargo pallet that can be carried in a cargo hold, divide its total weight, which includes the weight of the empty pallet and its tie-down devices, by its area in square feet. This load per square foot must be equal to or less than the floor load limit.

To help appreciate your aircraft’s weight-and-balance limits, consider the weight shift caused by one pilot leaving the cockpit while in flight, or an FBO ramp agent loading excessive weight in the nose. As part of the exercise, calculate the decrement to your aircraft’s OEI climb and high-altitude performance with an unintentional overload such as described in the earlier ASRS narrative. Ignorance can weigh heavily on mission outcome.

This article appears in the June 2016 issue of Business & Commercial Aviation magazine.