Anchorage, AK, USA
N275WA
Boeing MD-11F
The airline transport certificated flight crew was preparing to depart on a 14 CFR Part 91 ferry flight in an MD-11F freighter airplane with one of its three engines, the tail-mounted center engine inoperative, and the fuselage center landing gear (CLG) retracted. When the crew applied thrust for takeoff, the airplane pitched up, and the bottom of the tail struck the runway. The takeoff was aborted, and inspection disclosed minor damage to the lower empennage. In a letter to the NTSB investigator-in-charge, and in further discussions, company management stated that the crew had calculated the airplane's weight and balance based upon data for the CLG extended. They noted that they were unaware at the time of the incident flight that the airplane's aft limit of center of gravity (CG) moved significantly forward with the CLG retracted. They reported that the crew's CG calculations were reviewed by support, flight management, and dispatch staff prior to the flight. They also noted that while the crew was trained in two-engine ferry operations, their training did not include information regarding the CG shift with the CLG retracted. The operator said that while the appropriate weight and balance information was provided in an appendix to the approved Boeing Aircraft Flight Manual (AFM), the Boeing MD-11 Two-Engine Ferry Operations Manual the crew was using did not reference any change in the CG with the CLG retracted. The ferry manual also recommends in the pretakeoff checklist that the CLG be retracted. A review of the weight and balance data provided by the operator disclosed that the airplane was slightly aft of the zero fuel weight (ZFW) allowable center of gravity limit even when the calculations were computed using the more liberal CLG extended CG limits. When they were computed using the correct, CLG retracted data, the airplane's ZFW CG was approximately 3.2 per cent of mean aerodynamic chord (MAC) aft of the allowable limit. Discussions with Boeing staff disclosed that the MD-11 Two-Engine Ferry Manual was taken from the DC-10's. The DC-10 is a predecessor to the MD-11, and is similar in appearance and configuration, although the DC-10's fuselage is several feet shorter. The DC-10, however, does not have a forward shift in the aft CG limit when the CLG is retracted. Boeing personnel noted that although the MD-11's ferry manual recommends that the CLG be retracted for a two-engine ferry, they state that it is not essential. Boeing has issued a Flight Operations Bulletin (FOB), dated November 14, 2005, to MD-11 and DC-10 operators indicating that all DC-10 and MD-11 Two-Engine Ferry Manuals are out of date, and they cannot vouch for their accuracy. The FOB also has information on how the operators may obtain additional, current two-engine ferry performance/procedures information.
On October 8, 2004, about 1628 Alaska daylight time, a Boeing MD-11F airplane, N275WA, received minor damage when the lower empennage collided with the runway during an attempted takeoff from the Ted Stevens Anchorage International Airport, Anchorage, Alaska. The two airline transport certificated pilots were not injured. The flight was operated by World Airways, Inc., as a Title 14, CFR Part 91F ferry flight to the operator's maintenance base at Atlanta, Georgia. An IFR flight plan was filed, but not activated. Information received from the operator's vice president of flight operations disclosed that the flight crew had received an FAA-approved permit to ferry the empty, three engine airplane to Atlanta with the center (number two) engine inoperative. In order to enhance the climb performance and reduce drag, the crew elected to takeoff on runway 32 with the center landing gear (CLG) retracted, but calculated the airplane's center of gravity (CG) with the CLG extended. As calculated, using data for the CLG extended, the airplane's CG was in close proximity to the allowable aft CG limitations. However, when the CLG, (centered between the two main landing gear trucks) is retracted, the aft CG limit shifts forward. Using the correct, gear retracted CG data, the vice president of flight operations noted the actual takeoff CG was approximately 3.2 percent of mean aerodynamic chord (MAC) aft of the allowable limit. Upon application of full takeoff power and brake release, the airplane immediately rotated to an excessive nose-up attitude, and the lower empennage struck the runway. The crew aborted the takeoff, and taxied to parking. Examination of the airplane by the NTSB investigator-in-charge (IIC), and a review of the maintenance repair estimate produced by Boeing Commercial Airplanes, disclosed damage to two fuselage frames and at least two longerons had occurred. A section of lower fuselage skin, approximately 4 feet by 8 feet, also required replacement. The operator provided the NTSB IIC with a letter dated October 13, 2004, which stated, in part: "While some small computational errors may have occurred, the significant finding is a drastically curtailed CG envelope with center gear retracted. This is an oversight of World Airways on not providing operational personnel with this CG envelope during operations with the center gear retracted that is located in the Airplane Flight Manual, Appendix 8, page 2-4....Considering the center gear up CG limitations, the calculated ZFWCG (zero fuel weight center of gravity) and TOCG (take off center of gravity) were approximately 3.2% MAC and 2.3% MAC respectively, aft of their aft limits." A copy of the letter is included in the public docket of this report. World Airways also reported they replicated the incident flight in a Level D simulator (full motion and visual displays), and the simulator results mirrored the incident airplane's actions, with an abrupt pitch-up and tail strike. According to weight and balance data provided by Boeing, the aft limit TOCG with the CLG retracted at the incident airplane's weight, approximately 391,000 pounds, was 31.6% MAC, or 34% MAC with the CLG extended. The aft limit ZFWCG with the CLG retracted was approximately 28% MAC, or 31.2% MAC with the CLG extended. World Airways management personnel also stated in the October 13 letter that both pilots were check airmen trained in two-engine ferry procedures and that they had completed the weight and balance performance calculations using the data supplied for the CLG extended. The calculations were "...verified by flight operations management and support personnel. The dispatcher supervised weight and balance verification procedures." In a subsequent communication to the IIC, the Vice President of World's Flight Operations noted that while they only use highly experienced check airmen as captains for two engine ferry flights, and that there is a training program that addresses two engine ferry flights, the training program did not include training for CG calculations with the CLG retracted. Discussions with accident investigation and flight test staff at Boeing Aircraft yielded information similar to that provided by World Airways, in that an airplane loaded beyond the aft CG limit, with the CLG retracted, could pitch up abruptly and strike the tail upon an abrupt application of power. Boeing personnel also stated that the nose-up pitching moment would be exacerbated by the lack of thrust from the center engine, which is situated high in the tail of the airplane, and provides a partial nose-down thrust vector during takeoff. According to a Boeing senior flight safety investigator, Boeing staff also performed informal simulator exercises with the CLG retracted to see if a tail strike would occur. The Boeing investigator reported that by utilizing the recommended thrust setting technique in the Airplane Flight Manual (AFM), a tail strike did not occur at 34.0% MAC TOCG, and a takeoff weight of 391,250 pounds. The MD-11 AFM recommends that for takeoff, the airplane's brakes be locked, thrust set to 1.10 EPR (exhaust pressure ratio), and after brake release, a smooth and prompt application of power to the computed takeoff thrust EPR. A readout of the incident airplane's flight data recorder was accomplished by an NTSB vehicle recorder specialist. The EPR data for engines 1 and 3, the two operating engines, disclosed that during the takeoff attempt, the EPR's went from approximately 1.00 to 1.52 over a 12 second span. The vehicle recorder specialist noted as the EPR values for engines 1 and 3 began to rise, "...brake pressure and pedal position values indicate that the brakes were depressed at this time. After about 6 seconds, as the EPR values continued to increase, the brake pressure and pedal positions started to decrease and reached zero about 4 seconds later. At this time, the EPR values neared 1.6 and the ground speed and longitudinal began to increase. One second later, at SRN 4735, the nose gear parameter switched from compressed to not compressed. The EPR values began to decline after 2 seconds. The pitch parameter registered a maximum value 11.2 degrees..." A copy of the vehicle recorder specialist's factual report is in the public docket of this report. Copies of the original weight and balance computation documents the flight crew completed were provided by World to the IIC. The IIC and a Boeing weight and balance engineer reviewed the calculations, and noted that zero fuel weight (ZFW) center of gravity calculation placed the center of gravity slightly aft of the allowable limit, even when utilizing the less restrictive tables for the CLG extended. The Boeing engineer wrote: "The ZFW and TOW (take off weight) CG's are computed correctly. The computed ZFW CG from the Loading Manifest form is 32% and it appears to me to be plotted slightly aft of the ZFW limit (which I think is the case). At this point, additional center aux ballast fuel is required to move the ZFW CG forward of the AFT CG Limit, so for future flights, I would recommend that they add enough ballast fuel that the ZFW CG is clearly forward of rather than on or slightly aft of the CG Limit." Boeing staff sent two versions of the Two-Engine Ferry manual to the NTSB IIC, one for airplanes equipped with General Electric engines, and one for those equipped with Pratt and Whitney engines. The manual for airplanes with General Electric engines, MDC 91K0764, contained a page titled: "Weight and Balance Discussion." It notes, in part: "The aircraft Zero Fuel Weight (ZFW) must be reduced to the minimum possible to optimize aircraft takeoff and climb performance." It also contains the information that the weight and balance engineer "should be consulted quite early in the process and requested to compute the ZFW and CG using the methods described previously." The issue date on the copy provided to the IIC was August 28, 1992, with a revision date of January 11, 1994. The face of the copy noted: "This Copy Will Not Be Provided With Revision Service." The incident airplane was equipped with Pratt and Whitney engines. The Two-Engine Ferry Manual for MD-11 airplanes equipped with Pratt and Whitney engines, MDC 91K0763, did not contain the disclaimer regarding the revision service, nor the information about contacting the weight and balance engineer. Correspondence with Boeing staff disclosed that there should have been an identical disclaimer on the Pratt and Whitney version, and they were unaware why it was inadvertently omitted. Correspondence and relative documents from Boeing are included in the public docket of this report. The Boeing MD-11 Two-Engine Ferry Operations Manual pretakeoff checklist indicates that the CLG should be retracted for takeoff, but neither the manual or checklist alludes to a shift in the CG when the airplane is operated with the CLG retracted, nor does it direct the reader to specific CG calculation tables. During the IIC's discussions with Boeing personnel, it was discovered that the MD-11 Two-Engine Ferry Manual was taken from the DC-10's manual. The DC-10 was the predecessor to the MD-11, and is very similar in design. According to Boeing staff, unlike the MD-11, the DC-10 airplane does not have a forward shift in the aft center of gravity limit when the CLG is retracted. Boeing Safety staff have indicated that they will not revise the existing MD-11 Two-Engine Ferry Manual or checklist to reflect the change in CG when the CLG is retracted, or to modify the pretakeoff checklist to remove the recommendation to retract the CLG. They stated that they have assisted World Airways in modifying their in-house ferry procedures and manual, and that World has indicated they will not be conducting two-engine ferry flights with the CLG retracted. Boeing personnel stated that other operators may contact Boeing on an individual basis to receive guidance. They also stated that the Airplane Flight Manual (AFM) has an appendix that provides the appropriate weight and balance tables for operations with the CLG retracted. A Flight Operations Bulletin (FOB), Performance Bulletin MD-11-05-02, DC-10-05-01, applicable to all MD-11 and DC-10 airplanes, was issued by Boeing on November 14, 2005. The bulletin notes, in part, that the previous Two-Engine Ferry Operations Manuals were produced more than 10 years ago, that they have not been supported or revised for many years, and that Boeing does not vouch for the currency or accuracy of any of the existing two-engine ferry manuals. It also states that the manuals were to be used as supplemental information only, in conjunction with the Airplane Flight Manual and Flight Crew Operating Manuals. The bulletin directs operators desiring to conduct two-engine ferry flights to consult with their respective regulatory agencies for guidance/approval, and provides contact information for Boeing Flight Operations Engineering should additional performance data be required.
The operator's failure to provide sufficient weight and balance information to the flight crew, which resulted in a center of gravity aft of the limit, and a tail strike during the takeoff roll. Factors associated with the incident are the operator's improper training of the flight crew on two-engine ferry procedures, the flight crew's incorrect calculation of the center of gravity, the company dispatcher's failure to comply with the proper weight and balance procedures, and the airplane manufacturer's unclear/conflicting information contained in their Two-Engine Ferry Manual.
Source: NTSB Aviation Accident Database
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