7. There was no indication that the captain’s understanding of the rejected takeoff criteria was deficient; thus, the captain likely misspoke when she incorrectly stated the criteria in her pretakeoff briefing. 

8. The captain’s uncertainty as to whether to continue the takeoff suggests that her initial action to reject it did not result from a perception that the airplane was uncontrollable and could not fly. 

9. In the absence of evidence that the airplane was uncontrollable, the captain’s execution of a rejected takeoff for an unknown anomaly after the airplane’s speed had passed V1 was inconsistent with her training and standard operating procedures. 

10. The accident airplane’s uncommanded forward thrust, which accelerated the airplane at a time when the flight crew commanded full reverse thrust to decelerate the airplane, increased the severity of the accident because the uncommanded forward thrust substantially increased the airplane’s runway excursion speed. 

11. All four main landing gear tires on the airplane were operating while severely underinflated during the takeoff roll, which resulted in the tire failures. 

12. The accident airplane’s insufficient tire air pressure was due to Global Exec Aviation’s inadequate maintenance. 

13. Some operators are not sufficiently aware of the appropriate tire pressure check intervals for the airplanes in their fleets and are operating their airplanes with tires inflated below the aircraft maintenance manual replacement specifications. 

14. Aircraft maintenance manual (AMM) formats that refer to tire pressure checks as guidance information rather than required maintenance intervals and the lack of standardization of AMM formats with respect to the location of tire pressure check interval information do not provide sufficient emphasis on the criticality of checking and maintaining tire pressure. 

15. The Federal Aviation Administration’s legal interpretation that checking tire pressures on a Learjet 60 is preventive maintenance has an unintended negative effect on the safety of 14 Code of Federal Regulations (CFR) Part 135 operations because, according to the provisions of 14 CFR 43.3, a Learjet 60 pilot who is allowed to perform preventive maintenance, such as tire pressure checks, on the airplane for a flight operated under 14 CFR Part 91 is prohibited from performing the checks on the same airplane for a Part 135 flight. 

16. Tire pressure monitoring systems, which enable flight crews to easily verify tire pressures, provide safety benefits because the pressure loss rate of aircraft tires can result in tire pressures below acceptable operational values within only a few days, and such underinflation cannot be visually detected by flight crews. 

17. Learjet’s system safety analysis for and the Federal Aviation Administration’s review of the Learjet 60’s thrust reverser system modification and revised crew procedure were inadequate because they failed to effectively address an unsafe condition for all phases of flight, specifically, uncommanded forward thrust during a rejected takeoff. 

18. Had the Federal Aviation Administration adopted the procedures described in SAE International’s SAE ARP5150, Safety Assessment of Transport Airplanes in Commercial Service, to require a program for the monitoring and ongoing assessment of safety-critical systems, the FAA may have recognized, based on problems reported after previous incidents and an accident, that the Learjet 60’s thrust reverser system design was deficient and thus may have required appropriate modifications before this accident occurred. 

19. The Federal Aviation Administration’s 1993 certification of the Learjet 60 as a changed aeronautical product, which allowed the airplane’s equipment, systems, and installations to conform to some regulations applicable to the original 1966 certification, did not ensure the highest level of safety and allowed for deficiencies that would not likely have been present if the current regulations had applied. 

20. The accident pilots would have been better prepared to recognize the tire failure and to continue the takeoff if they had received realistic training in a flight simulator on the recognition of and proper response to tire failures occurring during takeoff. 

21. Because 14 Code of Federal Regulations Part 135 does not require that pilots in on‑demand turbojet operations have a minimum level of experience in airplane type, the pilots may lack adequate knowledge and skills in that airplane. 

22. The captain’s indecision in responding to the anomaly and her failure to follow standard operating procedures was the result of a combination of poor crew resource management skills, limited experience as a pilot-in-command in the Learjet 60, and, during the accident sequence in particular, her less than confident and assertive leadership in the cockpit. 

23. Although flight crew impairment related to diphenhydramine use or fatigue is possible, there is insufficient evidence to determine to what extent, if any, diphenhydramine use or fatigue may have affected the captain’s and the first officer’s performance. 

24. The captain’s passenger safety briefing contributed to the survival of two passengers. 

25. The tire design and testing requirements of 14 Code of Federal Regulations 25.733 may not adequately ensure tire integrity because they do not reflect the actual static and dynamic loads that may be imposed on tires both during normal operating conditions and after the loss of one tire, especially if the tires are operated at their load rating, and the requirements may not adequately account for tires that are operated at less-than-optimal conditions. 

26. A cockpit image recorder would have helped determine the precise speeds at which the accident airplane traveled and the flight crew’s responses to the anomaly, including flight and engine control inputs. 

PROBABLE CAUSE 

The National Transportation Safety Board determines that the probable cause of this accident was the operator’s inadequate maintenance of the airplane’s tires, which resulted in multiple tire failures during takeoff roll due to severe underinflation, and the captain’s execution of a rejected takeoff after V1, which was inconsistent with her training and standard operating procedures. 

Contributing to the accident were (1) deficiencies in Learjet’s design of and the Federal Aviation Administration’s (FAA) certification of the Learjet Model 60’s thrust reverser system, which permitted the failure of critical systems in the wheel well area to result in uncommanded forward thrust that increased the severity of the accident; (2) the inadequacy of Learjet’s safety analysis and the FAA’s review of it, which failed to detect and correct the thrust reverser and wheel well design deficiencies after a 2001 uncommanded forward thrust accident; (3) inadequate industry training standards for flight crews in tire failure scenarios; and (4) the flight crew’s poor crew resource management. 

RECOMMENDATIONS

New Recommendations 

The National Transportation Safety Board makes the following recommendations to the Federal Aviation Administration: 

1. Provide pilots and maintenance personnel with information that (1) transport‑category aircraft tires can lose up to 5 percent pressure per day, (2) it may take only a few days for such tires to reach an underinflation level below what the aircraft maintenance manual specifies for tire replacement, and (3) the underinflation level that would require tire replacement is not visually detectable. 

2. Require that all 14 Code of Federal Regulations Part 121, 135, and 91 subpart K operators perform tire pressure checks at a frequency that will ensure that the tires remain inflated to within aircraft maintenance manual-specified inflation pressures. 

3. Require that aircraft maintenance manuals specify, in a readily identifiable and standardized location, required maintenance intervals for tire pressure checks (as applicable to each aircraft). 

4. Allow pilots to perform tire pressure checks on aircraft, regardless of whether the aircraft is operating under 14 Code of Federal Regulations Part 91, Part 91 subpart K, or Part 135. 

5. Require tire pressure monitoring systems for all transport‑category airplanes. 

6. Identify the deficiencies in Learjet’s system safety analyses, both for the original Learjet 60 design and for the modifications after the 2001 accident, that failed to properly address the thrust reverser system design flaws related to this accident, and require Learjet to perform a system safety assessment in accordance with 14 Code of Federal Regulations 25.1309 for all other systems that also rely on air-ground signal integrity and ensure that hazards resulting from a loss of signal integrity are appropriately mitigated to fully comply with this regulation. 

7. Revise available safety assessment guidance (such as Advisory Circular 25.1309-1A) for manufacturers to adequately address the deficiencies identified in Safety Recommendation [6], require that designated engineering representatives and their Federal Aviation Administration (FAA) mentors are trained on this methodology, and modify FAA design oversight procedures to ensure that manufacturers are performing system safety analyses for all new or modified designs that effectively identify and properly mitigate hazards for all phases of flight, including foreseeable events during those phases (such as a rejected takeoff). 

8. Revise Federal Aviation Administration Order 8110.48 to require that the most current airworthiness regulations related to equipment, systems, and installations (14 Code of Federal Regulations 25.1309) are applied to all derivative design aircraft certificated as changed aeronautical products. 

9. Review the designs of existing derivative design aircraft that were certificated as changed aeronautical products against the requirements of the current revision of 14 Code of Federal Regulations 25.1309 and require modification of the equipment, systems, and installations to fully comply with this regulation. 

10. Define and codify minimum simulator model fidelity requirements for tire failure scenarios. These requirements should include tire failure scenarios during takeoff that present the need for rapid evaluation and execution of procedures and provide realistic sound and motion cueing. 

11. Once the simulator model fidelity requirements requested in Safety Recommendation [10] are implemented, require that simulator training for pilots who conduct turbojet operations include opportunities to practice responding to events other than engine failures occurring both near V1 and after V1, including, but not limited to, tire failures. 

12. Require that pilots who fly in 14 Code of Federal Regulations (CFR) Part 135 operations in aircraft that require a type rating gain a minimum level of initial operating experience, similar to that specified in 14 CFR 121.434, taking into consideration the unique characteristics of Part 135 operations. 

13. Require that pilots who fly in 14 Code of Federal Regulations (CFR) Part 135 operations in an aircraft that requires a type rating gain a minimum level of flight time in that aircraft type, similar to that described in 14 CFR 121.434, taking into consideration the unique characteristics of Part 135 operations, to obtain consolidation of knowledge and skills. 

14. Require that tire testing criteria reflect the actual static and dynamic loads that may be imposed on tires both during normal operating conditions and after the loss of one tire and consider less-than-optimal allowable tire conditions, including, but not limited to, the full range of allowable operating pressures and acceptable tread wear. 

Previously Issued Recommendations Reclassified in This Report: 

The following previously issued recommendations are classified in this report: 

Safety Recommendations A-09-55 through -58 and -60 to the FAA (previously classified “Open—Response Received”) are classified “Open—Acceptable Response.” 

A-09-55

Require Learjet to change the design of the Learjet 60 thrust reverser system in future-manufactured airplanes so that the reverse lever positions in the cockpit match the positions of the thrust reverser mechanisms at the engines when the thrust reversers stow. 

A-09-56

Once design changes are developed per Safety Recommendation A-09-55, require Learjet 60 operators to retrofit existing airplanes so that the reverse lever positions in the cockpit match the positions of the thrust reverser mechanisms at the engines when the thrust reversers stow. 

A-09-57

Require Learjet to develop and install improved aural or visual cues on future-manufactured Learjet 60 airplanes that would allow pilots to recognize an inadvertent thrust reverser stowage in a timely manner. 

A-09-58

Once improved aural or visual cues are developed per Safety Recommendation A-09-57, require Learjet 60 operators to install those cues on existing Learjet 60 airplanes. 

A-09-60

Evaluate the design of the thrust reverser controls and indications in Raytheon Hawker 1000 business jets for potential thrust reverser failure modes that are similar to those identified in Learjet 60 airplanes and implement necessary changes. 

Safety Recommendation A-09-59 to the FAA (previously classified “Open—Response Received”) is classified “Open—Acceptable Alternate Response.” 

A-09-59

Require that all Learjet 60 pilots receive training, for takeoff as well as landing phases of flight, on recognizing an inadvertent thrust reverser stowage, including the possibility that the stowage can occur when the requirements for deploying thrust reversers are not fully met, such as when the air/ground sensor squat switch circuits are damaged.