7263E 

E PLURIBUS UNUM

NATIONAL TRA S PORTATIO

N

B OAR D

SAFE T Y

N National Transportation Safety Board 

Washington, D.C. 20594 

Safety Recommendation 

Date: January 8, 2003

In reply refer to: A-02-36 through -51 

Honorable Marion C. Blakey 

Administrator 

Federal Aviation Administration 

Washington, D.C. 20591 

On January 31, 2000, about 1621 Pacific standard time, Alaska Airlines, Inc., flight 261, 

a McDonnell Douglas MD-83, N963AS, crashed into the Pacific Ocean about 2.7 miles north of 

Anacapa Island, California. The 2 pilots, 3 cabin crewmembers, and 83 passengers on board 

were killed, and the airplane was destroyed by impact forces. Flight 261 was operating as a 

scheduled international passenger flight under the provisions of 14 Code of Federal Regulations

(CFR) Part 121 from Lic Gustavo Diaz Ordaz International Airport (PVR), Puerto Vallarta, 

Mexico, to Seattle-Tacoma International Airport (SEA), Seattle, Washington, with an 

intermediate stop planned at San Francisco International Airport (SFO), San Francisco, 

California. Visual meteorological conditions prevailed for the flight, which operated on an 

instrument flight rules flight plan.1

The National Transportation Safety Board determined that the probable cause of this 

accident was a loss of airplane pitch control resulting from the in-flight failure of the horizontal 

stabilizer trim system jackscrew assembly’s acme nut threads. The thread failure was caused by 

excessive wear resulting from Alaska Airlines’ insufficient lubrication of the jackscrew 

assembly. Contributing to the accident were Alaska Airlines’ extended lubrication interval and 

the Federal Aviation Administration’s (FAA) approval of that extension, which increased the 

likelihood that a missed or inadequate lubrication would result in excessive wear of the acme nut 

threads, and Alaska Airlines’ extended end play check interval and the FAA’s approval of that 

extension, which allowed the excessive wear of the acme nut threads to progress to failure 

without the opportunity for detection. Also contributing to the accident was the absence on the 

McDonnell Douglas MD-80 (MD-80) of a fail-safe mechanism to prevent the catastrophic 

effects of total acme nut thread loss.2 

 1

 For more detailed information about this accident, see National Transportation Safety Board. Loss of Control 

and Impact with Pacific Ocean, Alaska Airlines Flight 261, McDonnell Douglas MD-83, N963AS, About 2.7 Miles 

North of Anacapa Island, California, January 31, 2000. Aircraft Accident Report NTSB/AAR-02/02. 

Washington, DC. 2

 Additional safety recommendations resulting from this accident investigation were addressed to the FAA in an 

October 1, 2001, safety recommendation letter (Safety Recommendations A-01-41 through -48). 
 2 

Accident Sequence3

The Safety Board determined that the longitudinal trim control system on the accident 

airplane was functioning normally during the initial phase of the accident flight, but that the 

horizontal stabilizer stopped responding to autopilot and pilot commands after the airplane 

passed through 23,400 feet. The pilots recognized that the longitudinal trim control system was 

jammed, but neither they nor the Alaska Airlines maintenance personnel could determine the 

cause of the jam.4

 When the acme screw and nut jammed, this prevented further movement of the 

horizontal stabilizer until the initial dive. The initial dive from 31,050 feet began when the jam 

between the acme screw and nut was overcome as a result of the operation of the primary trim 

motor.5 Release of the jam allowed the acme screw to pull up through the acme nut, causing the 

horizontal stabilizer leading edge to move upward, thus causing the airplane to pitch rapidly 

downward. The acme screw did not completely separate from the acme nut during the initial dive 

because the screw’s lower mechanical stop was restrained by the lower surface of the acme nut 

until just before the second and final dive about 10 minutes later. 

The Safety Board determined that the cause of the final dive was the low-cycle fatigue 

fracture of the torque tube, followed by the failure of the vertical stabilizer tip fairing brackets, 

which allowed the horizontal stabilizer leading edge to move upward significantly beyond what 

is permitted by a normally operating jackscrew assembly. The resulting upward movement of the 

horizontal stabilizer leading edge created an excessive upward aerodynamic tail load, which 

caused an uncontrollable downward pitching of the airplane from which recovery was not 

possible. 

Flight Crew Decision-Making 

Decision to Continue Flying Rather than Return to PVR 

Safety Board investigators considered several reasons that might explain the captain’s 

decision not to return immediately to PVR after he experienced problems with the horizontal 

stabilizer trim system during the climbout from PVR. 

Neither the Alaska Airlines MD-80 Quick Reference Handbook (QRH) Stabilizer 

Inoperative checklist nor the company’s QRH Runaway Stabilizer emergency checklist required 

landing at the nearest suitable airport if corrective actions were not successful. These checklist 

 3

 This section contains a brief summary of the Safety Board’s conclusions relating to the accident sequence. 

The basis for these conclusions are discussed in greater detail in the Board’s final report. 4

 The Board determined that the worn threads inside the horizontal stabilizer acme nut were incrementally 

sheared off by the acme screw and were completely sheared off during the accident flight. 5

 The horizontal stabilizer moved immediately after the autopilot was disconnected. Normally, the autopilot is 

disconnected by the pilot flying using the autopilot disconnect switch located on the outboard side of the control 

wheel. However, the autopilot will also disconnect when the primary trim control system is activated by either the 

control wheel trim switches or the longitudinal trim handles on the center pedestal. After the initial dive, the captain 

told the Alaska Airlines maintenance technician at Los Angeles International Airport (LAX), Los Angeles, 

California, “we did both the pickle switch [control wheel trim switches] and the suitcase handles [longitudinal trim 

handles] and it ran away full nose trim down [sic].” Based on this comment and those made by the Alaska Airlines 

LAX maintenance personnel immediately preceding the autopilot disconnect, it appears that the captain 

disconnected the autopilot when he activated the primary trim control system by using either the control wheel trim 

switches, the longitudinal trim handles, or both. 
 3 

procedures were the only stabilizer-related checklist procedures contained in the QRH, and the 

flight crew most likely followed these checklist procedures in their initial attempts to correct the 

airplane’s jammed stabilizer. 

The airplane’s takeoff weight of 136,513 pounds was well below the takeoff and climb 

limits for the departure runway, but it exceeded the airplane’s maximum landing weight of 

130,000 pounds. Because the airplane did not have an in-flight fuel dumping system, the airplane 

would have had to remain in flight for about 45 minutes after takeoff until enough fuel had 

burned to reduce the airplane’s weight by the 6,500 pounds needed to reach the airplane’s 

maximum landing weight. A return to PVR to execute an overweight landing would have 

required higher-than-normal approach speeds for landing and would have created additional 

workload and risk. An overweight landing at PVR would have been appropriate if the flight crew 

had realized the potentially catastrophic nature of the trim anomaly. However, in light of the 

airplane’s handling characteristics from the time of the initial detection of a problem to the initial 

dive, the flight crew would not have been aware that they were experiencing a progressive, and 

ultimately catastrophic, failure of the horizontal stabilizer trim system. 

The flight crew would have been aware that Alaska Airlines’ dispatch and maintenance 

control in SEA and LAX could be contacted by radio (via ground-based repeater stations) when 

the airplane neared the United States. However, even though the last horizontal stabilizer 

trimming movement was recorded by the flight data recorder (FDR) about 1349:51, the flight 

crew did not contact Alaska Airlines’ maintenance until shortly before the beginning of the 

cockpit voice recorder (CVR) transcript about 1549,6

 which suggests that control problems 

caused by the jammed horizontal stabilizer remained manageable for some time.7 Further, the 

positive aerodynamic effects of the higher cruise airspeed8 and fuel burn would have reduced the 

necessary flight control pressures to roughly 10 pounds9 and made the airplane easier to control. 

Therefore, the Safety Board concludes that, in light of the absence of a checklist requirement to 

land as soon as possible and the circumstances confronting the flight crew, the flight crew’s 

decision not to return to PVR immediately after recognizing the horizontal stabilizer trim system 

malfunction was understandable. 

Although they elected not to return to PVR, later in the flight the flight crew decided to 

divert to LAX, rather than continue to SFO, where the flight was originally scheduled to make an 

intermediate stop before continuing to SEA. Comments recorded by the CVR indicated that the 

 6

 The FDR indicates that a continuing series of radio transmissions began over the very-high frequency 2 

channel (used for all non-air traffic control radio transmissions) at 1521. These transmissions continued until the end 

of the FDR recording, which suggests that the flight crew began contacting Alaska Airlines maintenance personnel 

at this time. 

7

 The Safety Board notes that the 30-minute CVR recording did not capture the flight crew’s earlier 

troubleshooting efforts or the beginning of the flight crew’s discussions with maintenance personnel. A longer CVR 

recording that captured these events would have aided in this investigation. In Safety Recommendation A-99-16, the 

Board recommended that CVRs on all airplanes required to carry both a CVR and an FDR be capable of recording 

the last 2 hours of audio. This safety recommendation is currently classified “Open—Unacceptable Response.” 

8

 Although the flight plan called for a cruise speed of 283 knots calibrated airspeed, FDR data indicated that the 

airplane’s cruise speed increased, starting at 1424:30, eventually reaching 301 knots indicated airspeed (KIAS). 

9

 For almost 7 minutes after the autopilot disconnect, the airplane continued to climb at a much slower rate than 

before the disconnect, reaching about 31,050 feet by 1400:00. During this part of the ascent, the elevators were 

deflected in the airplane-nose-up direction between -1o and -3o, which, according to airplane performance 

calculations, would have required up to 50 pounds of combined pulling force on the control column(s). The 

calculations further indicated that, after leveling off at 31,050 feet, only about 30 pounds of pulling force would 

have been required to maintain level flight for the next 24 minutes while the airplane was flying at 280 KIAS. 
 4 

flight crew may have felt pressure from Alaska Airlines dispatch personnel to land in SFO.10

However, after discussing the malfunctioning trim system and current and expected weather 

conditions at SFO and LAX with Alaska Airlines dispatch and maintenance personnel, the 

captain decided to land at LAX rather than continue to SFO. The decision to divert to LAX was 

apparently based on several factors, including more favorable wind conditions at LAX 

(compared to a direct crosswind at SFO) that would reduce the airplane’s ground speed on 

approach and landing11 and the captain’s concern, expressed to Alaska Airlines dispatch 

personnel, about “overflying suitable airports.” The Safety Board concludes that the flight crew’s 

decision to divert the flight to LAX rather than continue to SFO as originally planned was 

prudent and appropriate. Further, the Safety Board concludes that Alaska Airlines dispatch 

personnel appear to have attempted to influence the flight crew to continue to SFO instead of 

diverting to LAX. 

Use of the Autopilot

After the flight crew had manually flown the airplane for almost 2 hours, the autopilot 

was engaged about 1547, disengaged at 1549:56, and re-engaged 19 seconds later. The autopilot 

remained engaged until 1609:16, just before the initial dive. No discussion on the CVR (which 

began at 1549) indicated why the autopilot was engaged in either instance. As discussed 

previously, the increased airspeed and reduced weight would have brought the airplane closer to 

a trimmed condition, thus allowing autopilot engagement to maintain altitude and heading while 

further troubleshooting was attempted. However, Alaska Airlines’ MD-80 QRH Stabilizer 

Inoperative checklist states, “do not use autopilot” if both trim systems are inoperative. In light 

of the autopilot’s inability to maintain trim (using the alternate trim motor) during the climbout 

from PVR and the flight crew’s subsequent unsuccessful attempts to manually activate the 

primary trim control system (using the primary trim motor), the flight crew should have known 

that both the alternate and the primary trim control systems were inoperative. Thus, the flight 

crew’s use of the autopilot was contrary to company procedures. 

Because the alternating current load meter registered electrical spikes when the crew 

attempted to activate the primary trim system, the flight crew should have realized that the 

primary trim motor was operational and that the system was jammed beyond the trim motor’s 

capability. Further, engagement of the autopilot, which would have been making automatic 

elevator corrections to the airplane’s mistrimmed condition, masked the airplane’s true condition 

from the flight crew. If the autopilot had subsequently been disconnected without one of the 

flight crewmembers holding the control wheel and making immediate corrective inputs, the 

airplane’s out-of-trim condition would have resulted in a severe pitch maneuver immediately 

 10 At 1552:02, after the captain had stated his intention to divert to LAX, Alaska Airlines dispatch personnel 

cautioned that if the flight landed at LAX rather than SFO, “we’ll be looking at probably an hour to an hour and a 

half [before the airplane could depart again] we have a major flow program going right now.” At 1552:41, the 

captain responded, “I really didn’t want to hear about the flow being the reason you’re calling us cause I’m 

concerned about overflying suitable airports.” At 1555:00, the captain commented to a flight attendant, “it just blows 

me away they think we’re gonna land, they’re gonna fix it, now they’re worried about the flow, I’m sorry this 

airplane’s [not] gonna go anywhere for a while...so you know.” After a flight attendant replied, “so they’re trying to 

put the pressure on you,” the captain stated, “well no, yea.” 

11 According to Alaska Airlines’ MD-80 QRH Stabilizer Inoperative checklist, 15o of flaps would have been the 

appropriate flap setting for the accident airplane’s approach and landing with a jammed stabilizer; this reduced flap 

setting would have increased approach speeds and required a corresponding increase in the amount of runway 

needed during landing. 
 5 

after the autopilot disconnect. Therefore, the Safety Board concludes that the flight crew’s use of 

the autopilot while the horizontal stabilizer was jammed was not appropriate.12

Configuration Changes

At 1615:56, after recovery from the initial dive, the captain told the air traffic controller 

that he wanted to “change my configuration, make sure I can control the jet and I’d like to do 

that out here over the bay if I may.” The captain then ordered extension of the slats at 1617:54 

and the flaps at 1618:05. The captain did not brief the first officer about what to expect or what 

to do if these configuration changes resulted in excessive flight control pressures or loss of 

control of the airplane. Further, the captain did not specify that the flaps should be extended at a 

slower-than-normal rate, which would have been a prudent precaution to minimize the 

possibility of the configuration change causing abrupt airplane movements that could be difficult 

to control. Nevertheless, at 1618:17, after the slats and flaps were extended, the captain noted 

that the airplane was “pretty stable right here.” The captain added that the airspeed needed to 

decrease to 180 KIAS (the airplane was then at 250 KIAS). Nine seconds later, at 1618:26, the 

captain ordered retraction of the slats and flaps, and the airspeed began to subsequently increase. 

It was not clear from the CVR recording why the captain ordered retraction of the slats and flaps 

and allowed the airspeed to increase nor did the CVR recording indicate any discussion about the 

possible effects of the slat and flap extension. 

The Safety Board notes that an airplane with flight control problems should be handled in 

a slow and methodical manner and that any configuration that would aid a landing should be 

maintained if possible. On the basis of the captain’s comment, the airplane was stable after the 

slat and flap extension at 1618:05. This configuration would have aided the approach and 

landing process. The Safety Board concludes that flight crews dealing with an in-flight control 

problem should maintain any configuration change that would aid in accomplishing a safe 

approach and landing, unless that configuration change adversely affects the airplane’s 

controllability. 

Activation of the Primary Trim Motor

At 1618:49, after the slats and flaps were retracted, the captain stated that he wanted to 

“get the nose up...and then let the nose fall through and see if we can stab it when it’s unloaded.” 

The first officer responded, “you mean use this again? I don’t think we should...if it can fly.” 

These statements suggest that the captain may have been indicating his intention to retry the 

primary trim system after reducing aerodynamic forces on the horizontal stabilizer. However, 

after the first officer’s statement at 1619:14, “I think if it’s controllable, we oughta just try to land 

it,” the captain abandoned his plan and responded, “ok let’s head for LA.” 

The Safety Board notes that the earlier repeated attempts to activate the primary trim 

system went well beyond what was called for in Alaska Airlines’ checklist procedures and 

ultimately precipitated the release of the jam of the acme screw and nut, resulting in the lower 

mechanical stop impacting the bottom of the nut. An additional attempt to use the trim switches 

at this point would not have been prudent. The severity of the initial dive changed the situation 

aboard the airplane to an emergency, which required a more deliberate and cautious approach. 

The Safety Board recognizes that, from an operational perspective, the flight crew could 

not have known the extent of airplane damage. Although flight crews are trained in jammed 

 12 The Safety Board also notes that the captain’s disconnection of the autopilot when he was the pilot not flying 

was also contrary to standard industry procedures. Normally, the pilot flying would use the autopilot disconnect 

switch on the control column and then assess the airplane’s controllability. 
 6 

stabilizer and runaway stabilizer scenarios, the loss of acme nut and screw engagement exceeded 

any events anticipated in emergency training scenarios, and the flight crew was not trained to 

devise or execute appropriate configurations and procedures to minimize further damage to the 

airplane or to prevent the accident. However, the flight crew’s earlier attempts to activate the trim 

motor and configuration changes may have worsened the situation. As previously discussed, the 

captain’s activation of the primary trim motor at 1609:16 precipitated the release of the jam and 

the initiation of the initial dive. However, it was not clear how many times previous to that the 

flight crew activated the primary trim motor nor was it clear whether or to what extent the prior 

activations hastened the release of the jam. Therefore, the Board could not determine the extent 

to which the activation of the primary trim motor played a role in causing or contributing to the 

accident. 

Adequacy of Current Guidance

The Safety Board notes that after the flight 261 accident, Boeing issued a flight 

operations bulletin outlining procedures to be followed in the event of an inoperative or 

malfunctioning horizontal stabilizer trim system. The bulletin advised flight crews to 

complete the flight crew operating manual (FCOM) checklist(s). Do not attempt additional 

actions beyond that contained in the checklist(s). If completing the checklist procedures does not 

result in operable trim system, consider landing at the nearest suitable airport. 

The Safety Board agrees that this advice is generally appropriate. However, the Board 

does not agree that the flight crew should merely “consider” landing at the nearest suitable 

airport if accomplishing the checklist items does not result in an operational trim system. In such 

a case, the flight crew should always land at the nearest suitable airport as expeditiously and 

safely as possible. Further, the bulletin provides additional information regarding the possibility 

that repeated or continuous use of the trim motors may result in thermal cutoff and states that the 

motor may reset after a cooling period. The Board is concerned that this additional information 

addressing repeated or continuous use of the trim motors may weaken or confuse the initial 

guidance to refrain from attempting troubleshooting measures beyond those specified in the 

checklist procedures. 

The Safety Board concludes that, without clearer guidance to flight crews regarding 

which actions are appropriate and which are inappropriate in the event of an inoperative or 

malfunctioning flight control system, pilots may experiment with improvised troubleshooting 

measures that could inadvertently worsen the condition of a controllable airplane. Accordingly, 

the Safety Board believes that the FAA should issue a flight standards information bulletin 

directing air carriers to instruct pilots that in the event of an inoperative or malfunctioning flight 

control system, if the airplane is controllable they should complete only the applicable checklist 

procedures and should not attempt any corrective actions beyond those specified. In particular, in 

the event of an inoperative or malfunctioning horizontal stabilizer trim system, after a final 

determination has been made in accordance with the applicable checklist that both the primary 

and alternate trim systems are inoperative, neither the primary nor the alternate trim motor 

should be activated, either by engaging the autopilot or using any other trim control switch or 

handle. Pilots should further be instructed that if checklist procedures are not effective, they 

should land at the nearest suitable airport. The Safety Board also believes that the FAA should 

direct all Certificate Management Offices to instruct inspectors to conduct surveillance of airline 

dispatch and maintenance control personnel to ensure that their training and operations directives 

provide appropriate dispatch support to pilots who are experiencing a malfunction threatening 

safety of flight and instruct them to refrain from suggesting continued flight in the interest of 

airline flight scheduling. 
 7 

Lubrication of the Jackscrew Assembly 

Alaska Airlines’ Lubrication Interval Extension

Douglas DC-9 (DC-9) test results and Douglas DC-8 service history indicated that 

frequent lubrication of the jackscrew assembly would allow the acme screw to meet its original 

design life of 30,000 flight hours. DC-9 certification documents, including Douglas Process 

Standard 3.17-49 (issued August 1, 1964), specified a lubrication interval for the jackscrew 

assembly of 300 to 350 flight hours. However, the 300- to 350-flight-hour recommended 

lubrication interval for the DC-9 was not contained in the manufacturer’s initial on-aircraft 

maintenance planning (OAMP) documents for the DC-9 or the MD-80. Instead, those documents 

specified a lubrication interval of 600 to 900 flight hours. 

In 1987, Alaska Airlines’ lubrication interval for horizontal stabilizer components, 

including the jackscrew assembly, was every 500 flight hours, consistent with the manufacturer’s 

recommendation in the Maintenance Review Board (MRB) report and OAMP documents 

derived from Maintenance Steering Group (MSG)-2 guidance, which recommended a lubrication 

interval of 600 to 900 flight hours. In 1988, Alaska Airlines’ lubrication interval increased to 

every 1,000 flight hours (a 100 percent increase); in 1991, to every 1,200 flight hours (an 

additional 20 percent increase); and, in 1994, to every 1,600 flight hours (an additional 

33 percent increase). In 1996, the interval was changed to 8 months with no specified flight-hour 

limit. Based on fleet utilization at the time, 8 calendar months equated to about 2,550 flight 

hours, an additional increase of greater than 59 percent. Thus, at the time of the accident, Alaska 

Airlines’ lubrication interval for the jackscrew assembly was more than 400 percent greater than 

it was in 1987. 

The investigation did not determine what type of information, if any, was presented as 

justification for the lubrication interval extensions in 1988, 1991, and 1994. However, according 

to the FAA principal maintenance inspector (PMI) for Alaska Airlines, who reviewed and 

accepted the 1996 interval extension, Alaska Airlines presented documentation of the 

manufacturer’s recently extended recommended interval as justification for its increase. (The 

extended recommended lubrication interval in the MRB report and OAMP document derived 

from the MSG-3 guidance was every C check, or every 3,600 flight hours.) 

Testimony at the Safety Board’s public hearing and Boeing documents indicated that the 

original design engineers’ recommended lubrication interval was not considered during the 

MRB-3 decision-making process regarding the extension of the manufacturer’s recommended 

interval. Further, Boeing design engineers were not consulted about nor aware of the extended 

3,600-flight-hour MRB-3 recommended lubrication interval. Although Alaska Airlines’ extended 

lubrication and end play check intervals have now been superseded by the 650-flight-hour 

lubrication and 2,000-flight-hour end play check intervals specified in Airworthiness Directive 

(AD) 2000-15-15, the Board is concerned that there is no mechanism in place to prevent similar 

unsafe interval extensions for other maintenance tasks. This issue and associated safety 

recommendations are discussed further later in this letter. 

Safety Implications of Lubrication Interval Extension

As grease is used in a system, it loses its effectiveness over time and requires 

replacement. Longer lubrication intervals increase the likelihood that a missed or inadequate 

lubrication will result in excessive wear. Conversely, shorter lubrication intervals increase the 

likelihood that, even if a lubrication is missed or inadequately performed, the existing grease will 

remain effective until the next scheduled lubrication. 
 8 

The Safety Board notes that at the time of Alaska Airlines’ increase to an 8-month 

lubrication interval, it was the only U.S. airline that had a calendar-time lubrication interval with 

no accompanying flight-hour limit and no specification, “whichever comes first.” A 

calendar-time lubrication interval can degrade the margin of safety because wear is directly 

related to aircraft usage, or flight hours, and not to calendar time. Also, a purely calendar-based 

interval does not account for increases in flight hours that result from increased airplane 

utilization. Conversely, intervals based on flight hours, or on calendar time with an 

accompanying flight-hour time limit and the proviso, “whichever comes first,” ensure that the 

flight-hour limit will not be exceeded as a result of increased airplane utilization. Thus, unless a 

maximum utilization is also specified, calendar-time intervals are inappropriate for certain tasks, 

such as lubrication or inspections for fatigue, when component deterioration is related to usage, 

not time.13 

In sum, at the time of the accident, Boeing’s recommended lubrication interval for the 

MD-80 jackscrew assembly was every 3,600 flight hours, about 4 to 6 times longer than 

Douglas’ original recommendation in the MSG-2 OAMP document of every 600 to 900 flight 

hours. Alaska Airlines’ lubrication interval for the MD-80 jackscrew assembly—although it was 

still less than the manufacturer’s recommended interval in the MSG-3 OAMP document of 

3,600 flight hours—was about 3 to 4 times longer than Douglas’ originally recommended 

lubrication interval, resulting in a significant decrease in the MD-80 fleet’s ability to tolerate 

missed or inadequate lubrications. The Safety Board notes that the negative safety implications 

of the ongoing lubrication interval extensions were magnified by the simultaneous ongoing 

extensions of the end play check interval in that there would be fewer and fewer opportunities to 

discover and address any excessive wear resulting from lubrication deficiencies. 

Therefore, the Safety Board concludes that Alaska Airlines’ extensions of its lubrication 

interval for its MD-80 horizontal stabilizer components and the FAA’s approval of these 

extensions, the last of which was based on Boeing’s extension of the recommended lubrication 

interval, increased the likelihood that a missed or inadequate lubrication would result in 

excessive wear of jackscrew assembly acme nut threads and, therefore, was a direct cause of the 

excessive wear and contributed to the Alaska Airlines flight 261 accident. 

Adequacy of Lubrication Procedures

The horizontal stabilizer lubrication procedure specifies that, after the access doors are 

opened, grease is to be applied to the acme nut grease fitting under pressure. The procedure 

further specifies the brush application of a light coat of grease to the acme screw threads and 

operation of the trim system through its full range of travel to distribute the grease over the 

length of the acme screw. 

Safety Board investigators observed maintenance personnel from two MD-80 operators 

perform jackscrew assembly lubrications and discussed the lubrication procedure with those and 

other maintenance personnel from those operators. Investigators noted many differences in the 

methods used by the personnel to accomplish certain steps in the lubrication procedure, including 

the manner in which grease was applied to the acme nut fitting and screw and the number of 

times the trim system was cycled to distribute the grease. Several of the methods observed by or 

reported to investigators did not involve application of grease to the entire length of the acme 

screw and cycling the trim system several times. 

 

 13 In contrast, some maintenance tasks are intended to prevent or control conditions (such as corrosion or 

deterioration based on aging) that are based solely on the passage of time. Such tasks might appropriately be tied to 

calendar-time intervals without any flight-hour limits. 
 9 

Laboratory demonstrations designed to compare the effectiveness of various methods of 

lubricating the jackscrew assembly found that application of a complete coating of grease over 

all exposed threads, filling the thread valleys, followed by the cycling of the trim system several 

times, maximized the distribution of the grease over the length of the acme screw. In contrast, the 

observations and demonstrations established that applying grease only through the acme nut 

grease fitting and then cycling the trim system several times did not distribute an adequate 

amount of grease over the remainder of the acme screw. Although both methods of grease 

application are specified in the lubrication procedure, if a mechanic mistakenly believed that 

lubricating only through the acme nut grease fitting was adequate, the acme screw and nut would 

receive insufficient lubrication. 

The extent to which these deficiencies in the lubrication procedure may have played a 

role in the inadequate lubrication of the accident jackscrew assembly could not be determined. 

However, in an October 1, 2001, safety recommendation letter, the Safety Board expressed its 

concern that the current lubrication procedure was not adequate to ensure consistent and 

thorough lubrication of the jackscrew assembly by all operators and issued Safety 

Recommendation A-01-41, which asked the FAA to require Boeing to “revise the lubrication 

procedure for the horizontal stabilizer trim system of Douglas DC-9, McDonnell Douglas 

MD-80/90, and Boeing 717 [717] series airplanes to minimize the probability of inadequate 

lubrication.” In a December 12, 2001, letter, the FAA responded that it agreed with the intent of 

Safety Recommendation A-01-41 and that it was “working with the Boeing Commercial 

Airplane Group to rewrite the lubrication procedures to the optimal standard.” On June 14, 2002, 

the Safety Board classified Safety Recommendation A-01-41 “Open—Acceptable Response.” 

After the issuance of Safety Recommendation A-01-41, Safety Board investigators 

continued to evaluate the adequacy of the current lubrication procedure and identified three 

additional areas that should be addressed. 

 First, the Safety Board is concerned about the wear debris produced as a result of the 

wear process and other foreign debris that may accumulate in the grease over time. Because 

material that infiltrates the working grease reduces its lubricating effectiveness, it is desirable to 

flush out these materials before fresh grease is added (that is, to completely replace used, less 

effective grease with fresh, more effective grease).14 The Board notes that the current jackscrew 

lubrication procedure does not stipulate the removal of used grease from the acme screw before 

the application of fresh grease.15 The Board is aware that Boeing is developing a revised 

lubrication procedure for the jackscrew assembly that includes removal of the used grease before 

application of the fresh grease. Although the revised procedure, if properly performed, should 

improve the effectiveness of the lubrication, it is not yet known whether the FAA will require the 

use of this improved procedure. 

The Safety Board concludes that when lubricating the jackscrew assembly, removal of 

degraded grease from the acme screw before application of fresh grease will increase the 

effectiveness of the lubrication. Therefore, the Safety Board believes that as part of the response 

to Safety Recommendation A-01-41, the FAA should require operators of DC-9, MD-80/90, and 

 

 14 This finding was further supported by interviews with representatives of the largest manufacturer of 

nonaviation jackscrew assemblies, Nook Industries. Nook representatives told Safety Board investigators that 

purging old grease out of the jackscrew assembly, and then refreshing the acme screw with new grease, increases the 

life of the acme nut. 

 15 The current jackscrew lubrication procedure does call for the application of new grease into the nut, via the 

grease fitting, until grease is observed extruding from the nut; however, this does not remove the old grease from the 

acme screw surface or a large portion of the nut. 
 10 

717 series airplanes to remove used grease from the jackscrew assembly acme screw and flush 

degraded grease and particulates from the acme nut before applying fresh grease. 

Second, Safety Board investigators noted that when they attempted to perform the 

lubrication procedure, it was difficult to insert a hand through the access panel openings because 

of their size.16 They further noted that after a hand was inserted, it blocked the view of the 

jackscrew assembly, requiring the task to be accomplished primarily by “feel.” The Board is 

aware that Boeing is developing a modification for an expanded access panel. 

The Safety Board concludes that a larger access panel would facilitate the proper 

accomplishment of the jackscrew assembly lubrication task. Therefore, the Safety Board believes 

that, as part of the response to Safety Recommendation A-01-41, the FAA should require 

operators of DC-9, MD-80/90, and 717 series airplanes, in coordination with Boeing, to increase 

the size of the access panels that are used to accomplish the jackscrew assembly lubrication 

procedure. 

Third, this investigation has highlighted the need for improved methods for ensuring that 

jackscrew assembly lubrications are accomplished properly at scheduled lubrication intervals. 

Currently, the lubrication procedure is generally performed and signed off by a single 

maintenance technician, and that technician’s work is not required to be inspected. 

Although the Safety Board cannot be certain that such a requirement would have 

prevented the Alaska Airlines flight 261 accident, the Safety Board concludes that if the 

jackscrew assembly lubrication procedure were a required inspection item for which an 

inspector’s signoff is needed, the potential for unperformed or improperly performed lubrications 

would be reduced. Therefore, because of the critical importance of adequately lubricating the 

jackscrew assembly and the potentially catastrophic effects of excessive acme nut wear resulting 

from insufficient lubrication, the Safety Board believes that the FAA should establish the 

jackscrew assembly lubrication procedure as a required inspection item that must have an 

inspector’s signoff before the task can be considered complete. 

Monitoring Acme Nut Thread Wear 

Because the MD-80 jackscrew assembly’s structural function is critical to the safety of 

flight, and that structural function cannot be maintained without proper acme nut and screw 

thread engagement, it is essential that acme nut thread wear be regularly monitored. The failure 

to adequately monitor acme nut thread wear may result in continued operation of an airplane 

with excessive nut thread wear. As demonstrated by the Alaska Airlines flight 261 accident, 

because no other structure performs the function of the jackscrew assembly, the loss of acme nut 

and screw engagement as a result of excessive wear will most likely have catastrophic results. 

In-service acme nut thread wear is monitored by performing an on-wing end play check 

procedure at specified intervals. Before 1967, there were no required periodic inspections. The 

jackscrew assembly had an expected service life of 30,000 hours, at which time the end play 

measurement was expected to be 0.0265 inch. However, as a result of higher-than-expected wear 

rates reported in 1966, in 1967, Douglas developed the end play check procedure and increased 

the maximum permissible end play measurement at which a jackscrew assembly could remain in 

service to 0.040 inch. (The minimum end play measurement remained at 0.003 inch.)17 In All 

 

 16 Two access panel openings (one 4 inches by 6 inches and the other 8 inches by 7 inches) are generally used 

to accomplish the lubrication procedure. 

 17 As a result of these higher-than-expected wear rates, Douglas also changed some of the materials 

specifications and manufacturing processes for the acme screw (increased heat treating and nitriding) to reduce 
 11 

Operators Letter 9-48A, Douglas stated that jackscrew assemblies could remain in service as 

long as the end play measurement remained within these tolerances. 

Alaska Airlines’ Preaccident End Play Check Intervals 

Alaska Airlines has consistently required end play checks at every other C check. 

However, the length of its C-check interval has changed over time. As a result of those C-check 

interval changes, between 1985 and 1996 Alaska Airlines increased its end play check interval 

by almost 200 percent. 

In 1985, when Alaska Airlines’ MD-80 maintenance program was initially approved by 

the FAA, C checks were conducted every 2,500 flight hours; therefore, end play checks were 

performed every 5,000 flight hours. By July 1988, C-check intervals had been extended to 

13 months, with no corresponding flight-time limit; therefore, end play checks were performed 

every 26 months, which was approximately 6,400 flight hours, based on the airplane utilization 

rate at that time. Although FAA approval would have been required for this C-check interval 

extension, no information was available regarding what documentation, if any, Alaska Airlines 

presented to the FAA to justify the extension. 

In 1996, Alaska Airlines’ C-check interval was extended to 15 months; therefore, end 

play checks were only required every 30 months. Because Alaska Airlines’ airplane utilization 

had also increased, this 30-month interval was equivalent to approximately 9,550 flight hours. 

Alaska Airlines sought and obtained advance FAA approval for this C-check interval extension. 

Alaska Airlines’ director of reliability and maintenance programs testified at the public hearing 

that Alaska Airlines had presented the FAA with a data analysis package based on the 

maintenance histories of five sample airplanes to justify the C-check interval extension. He 

indicated that individual maintenance tasks tied to C-check intervals (such as the end play check) 

were not separately considered in connection with the extension.18 Thus, the FAA’s approval of 

the 1996 C-check extension also effectively constituted approval to extend the end play check 

from 26 to 30 months, a 15 percent increase, and, more importantly, a 55 percent increase in 

flight-hour intervals, from approximately 6,400 to 9,550 flight hours. 

Although Alaska Airlines’ extended 30-month end play check interval was consistent 

with the manufacturer’s recommended calendar-time limit, the resulting 9,550-flight-hour 

interval far exceeded the manufacturer’s recommended flight-hour interval, which at that time 

was 7,000 or 7,200 flight hours (depending on whether MSG-2 or MSG-3 guidance was used). 

At that time, Alaska Airlines had the second highest end play check interval of all operators of 

DC-9, MD-80/90, and 717 series airplanes. Further, Alaska Airlines was the only U.S. carrier at 

that time that did not have an accompanying flight-hour limit with the specification, “whichever 

comes first,” for its end play check interval. As discussed previously in connection with Alaska 

Airlines’ lubrication interval, use of only a calendar-time interval does not account for increased 

airplane utilization rates and could result in a lower safety margin than intended. 

The Safety Board notes that, because of Alaska Airlines’ extended end play check 

interval of 30 months, or about 9,550 flight hours, after the accident airplane’s last end play 

check in September 1997, the jackscrew assembly would not have been required to undergo 

 

wear. 

 18 However, based on the history of maintenance discrepancies noted for the five sample airplanes, two 

maintenance tasks were identified as inappropriate for extension, and these tasks were converted to stand-alone 

items to be performed at shorter intervals. The two maintenance tasks identified as requiring shorter intervals based 

on service history were (1) lubrication of the bent-up, trailing-edge wing doors and (2) lubrication of the bearings 

and bushings in the elevator hinges. 
 12 

another end play check until March 2000. Between the time that the 0.040- and 0.033-inch end 

play measurements were recorded in September 1997 and the time of the accident, the airplane 

had been flown for 28 months, nearly 9,000 flight hours. During this time, the acme nut thread 

wear progressed to failure. 

In light of what has been learned in this investigation, it is now apparent that the 

manufacturer’s previously recommended end play check intervals of 7,000 or 7,200 flight hours 

were not adequate.19 Nonetheless, the Safety Board notes that if Alaska Airlines had not extended 

its end play check interval to beyond the recommended interval, the airplane would have been 

required to undergo an end play check at least 1,800 to 2,000 flight hours before the accident, 

and the excessive end play could have been identified at that time. Thus, the Safety Board 

concludes that Alaska Airlines’ extension of the end play check interval and the FAA’s approval 

of that extension allowed the accident acme nut threads to wear to failure without the opportunity 

for detection and, therefore, was a direct cause of the excessive wear and contributed to the 

Alaska Airlines flight 261 accident. 

Adequacy of Existing Process for Establishing Maintenance Task Intervals

The Safety Board is concerned that the absence of any significant maintenance history 

pertaining to the jackscrew assembly was apparently considered by Alaska Airlines and the FAA 

as sufficient justification to extend the end play check interval as part of the C-check interval 

extension. In general, the absence of maintenance history should not be considered adequate 

justification to extend the interval for the performance of a critical maintenance task. Any 

significant maintenance change associated with a critical flight control system should be 

independently analyzed and supported by technical data demonstrating that the proposed change 

will not present a potential hazard. Therefore, any maintenance task change related to the 

jackscrew assembly, which is an essential element of a critical flight control system, should be 

handled in this manner. 

The Safety Board is further concerned that the MSG and MRB-based process by which 

manufacturers develop initial and revised recommended maintenance task intervals resulted in 

significant extensions of both the lubrication and end play check intervals without any such 

analysis or support. Testimony at the Safety Board’s public hearing and Boeing documents 

indicated that Douglas’ original recommended lubrication interval of 600 to 900 flight hours was 

not considered during the MSG-3 decision-making process to extend the recommended interval 

to 3,600 hours. Further, Boeing design engineers were not consulted about nor aware of the 

escalated lubrication interval specified in the MSG-3 documents. The FAA’s MD-80 MRB 

chairman testified at the public hearing that the escalation of C-check intervals in the MSG-3 

MD-80 MRB did not involve a task-by-task analysis of each task (such as the jackscrew 

lubrication task and end play check) that would be affected by the changed interval. 

The Safety Board concludes that Alaska Airlines’ end play check interval extension 

should have been, but was not, supported by adequate technical data to demonstrate that the 

extension would not present a potential hazard. The Safety Board further concludes that the 

existing process by which manufacturers revise recommended maintenance task intervals and by 

which airlines establish and revise these intervals does not include task-by-task engineering 

analysis and justification and, therefore, allows for the possibility of inappropriate interval 

extensions for potentially critical maintenance tasks. In addition, the Board notes that the FAA 

plays a limited role in this process compared to the role it plays in the initial certification process. 

 

 19 This issue is discussed further later in this letter. 
 13 

Therefore, the Safety Board believes that the FAA should review all existing maintenance 

intervals for tasks that could affect critical aircraft components and identify those that have been 

extended without adequate engineering justification in the form of technical data and analysis 

demonstrating that the extended interval will not present any increased risk and require 

modification of those intervals to ensure that they (1) take into account assumptions made by the 

original designers, (2) are supported by adequate technical data and analysis, and (3) include an 

appropriate safety margin that takes into account the possibility of missed or inadequate 

accomplishment of the maintenance task. In conducting this review, the FAA should also 

consider original intervals recommended or established for new aircraft models that are 

derivatives of earlier models and, if the aircraft component and the task are substantially the 

same and the recommended interval for the new model is greater than that recommended for the 

earlier model, treat such original intervals for the derivative model as “extended” intervals. The 

Safety Board further believes that the FAA should conduct a systemic industrywide evaluation 

and issue a report on the process by which manufacturers recommend and airlines establish and 

revise maintenance task intervals and make changes to the process to ensure that, in the future, 

intervals for each task (1) take into account assumptions made by the original designers, (2) are 

supported by adequate technical data and analysis, and (3) include an appropriate safety margin 

that takes into account the possibility of missed or inadequate accomplishment of the 

maintenance task. 

The Safety Board also believes that the FAA should require operators to supply the FAA, 

before the implementation of any changes in maintenance task intervals that could affect critical 

aircraft components, technical data and analysis for each task demonstrating that none of the 

proposed changes will present any potential hazards, and obtain written approval of the proposed 

changes from the PMI and written concurrence from the appropriate FAA Aircraft Certification 

Office. 

Adequacy of Current End Play Check Intervals

The Safety Board recognizes that the FAA acted promptly following the Alaska Airlines 

flight 261 accident by issuing ADs that shortened the end play check interval to 2,000 flight 

hours. However, evidence collected during this investigation suggests that acme nut thread wear 

at a higher-than-expected rate could allow a potentially dangerous level of wear to occur in less 

than 2,000 hours. 

Two jackscrew assemblies installed sequentially on the same Hawaiian Airlines DC-9 

airplane that were removed because of high end play measurements were found to have worn at 

an unprecedented rate. The first acme nut had an approximate wear rate of 0.015 inch per 

1,000 flight hours, and the second had an approximate wear rate of 0.008 inch per 1,000 flight 

hours. These wear rates are about 15 and 8 times greater, respectively, than the expected wear 

rate of about 0.001 inch per 1,000 flight hours. This accelerated wear was attributed to the 

presence of grit-blasting material that had been introduced inadvertently into the jackscrew 

assembly and become embedded in the grease on the acme screw. 

If the first excessively worn jackscrew assembly had measured just within, rather than 

beyond, the 0.040-inch limit at the last end play check, and assuming the acme nut threads had 

continued to wear at an approximate rate of 0.015 inch per 1,000 flight hours, the end play 

measurement would have been about 0.069 inch by the time of its next scheduled end play check 

2,000 flight hours later. Further, assuming that this end play check was either missed or 

improperly accomplished and the jackscrew assembly remained in service despite an end play of 

0.069 inch and continued to wear at the same rate, the end play measurement would have been 

about 0.099 inch by the time of the next scheduled end play check. Moreover, there is no basis 

for assuming that the wear rate of this Hawaiian Airlines jackscrew assembly represents the 

maximum possible acme nut thread wear rate. Just as the wear rate and wear mechanism on the 
 14 

Hawaiian Airlines jackscrew assembly was unprecedented and unanticipated, there may be other 

unprecedented and unanticipated wear rates and mechanisms that could also result in excessive 

or accelerated wear. Therefore, it is possible that acme nut threads could wear at an even faster 

rate than the Hawaiian Airlines acme nut threads, possibly even to catastrophic limits20 in 

2,000 flight hours. 

To establish an appropriately conservative end play check interval, the uncertainties 

regarding possible wear mechanisms and the maximum possible wear rate must be considered in 

addition to the significant possibility of inaccurate end play measurements. The Safety Board 

notes that, when failure mechanisms are known and clearly defined, standard industry practice to 

ensure a damage tolerant design is to have a safety margin that allows for two complete 

inspection cycles before the predicted failure time.21 Thus, even if one inspection is missed or 

inadequately performed, there will be at least one other opportunity to detect and correct the 

condition. However, when uncertainties exist in the failure mechanism, as in the case of acme 

nut thread wear, standard industry practice is to increase the safety margin to account for the 

reduced level of confidence in the predicted failure time. 

In a June 26, 2001, letter to the Safety Board, the FAA stated that it believed the 

2,000-flight-hour interval provided an acceptable level of safety, citing the “robust design” of the 

acme nut and the fact that it could safely carry normal flight loads even when worn beyond 

0.080 inch. The FAA also stated that the 650-flight-hour inspection and lubrication interval in 

AD 2000-15-15 provided frequent opportunities (in addition to the end play check every 

2,000 flight hours) to detect wear debris and, therefore, possible excessive wear. Nonetheless, the 

Board is concerned that significantly higher-than-expected wear—wear even greater than that of 

the Hawaiian Airlines jackscrew assemblies—could result from foreign-object contamination, 

such as grit blast, or from other factors that have not yet been identified. 

The Safety Board concludes that, because of the possibility that higher-than-expected 

wear could cause excessive wear in less than 2,000 flight hours and the additional possibility that 

an end play check could be not performed or improperly performed, the current 2,000-flight-hour 

end play check interval specified in AD 2000-15-15 may be inadequate to ensure the safety of 

the DC-9, MD-80/90, and 717 fleet. Therefore, the Safety Board believes that, pending the 

incorporation of a fail-safe mechanism in the design of the DC-9, MD-80/90, and 717 horizontal 

stabilizer jackscrew assembly, as recommended in Safety Recommendation A-02-49 in this 

report, the FAA should establish an end play check interval that (1) accounts for the possibility of 

higher-than-expected wear rates and measurement error in estimating acme nut thread wear and 

(2) provides for at least two opportunities to detect excessive wear before a potentially 

catastrophic wear condition becomes possible. 

To establish an appropriate end play check interval, it is necessary to monitor end play 

measurements over time to identify any excessive or unanticipated wear rates and to continue 

evaluating the reliability and validity of end play measurements. Therefore, the FAA, Boeing, 

maintenance facilities that overhaul jackscrew assemblies, and operators should closely evaluate 

 

 20 According to the Safety Board’s study of thread stress and deformation, the acme nut threads will begin to 

deflect and begin the process of sliding over the acme screw threads at a wear level of about 0.093 inch. 

 21 According to the FAA’s Damage Tolerance Assessment Handbook, Volume I, issued in February 1999, 

“damage tolerance refers to the ability of the design to prevent structural cracks from precipitating catastrophic 

fracture when the airframe is subjected to flight or ground loads. Transport category airframe structure is generally 

made damage tolerant by means of redundant (‘fail safe’) designs for which the inspection intervals are set to 

provide at least two inspection opportunities per number of flights or flight hours it would take for a visually 

detectable crack to grow large enough to cause a failure in flight.” Although this refers to cracks, the Safety Board 

notes that the damage tolerance principles can be applied to acme nut wear. 
 15 

the measurement data currently being reported pursuant to AD 2000-15-15. The Safety Board 

concludes that the continued collection and analysis of end play data are critical to monitoring 

acme nut thread wear and identifying excessive or unexpected wear rates, trends, or anomalies. 

Therefore, the Safety Board believes that the FAA should require operators to permanently (1) 

track end play measurements according to airplane registration number and jackscrew assembly 

serial number, (2) calculate and record average wear rates for each airplane based on end play 

measurements and flight times, and (3) develop and implement a program to analyze these data 

to identify and determine the cause of excessive or unexpected wear rates, trends, or anomalies. 

The Safety Board further believes that the FAA should require operators to report this 

information to the FAA for use in determining and evaluating an appropriate end play check 

interval. 

Deficiencies of Jackscrew Assembly Overhaul Procedures and Practices 

The accident jackscrew assembly was never overhauled nor was it required to be. 

However, to determine the adequacy of maintenance and inspection procedures applicable to all 

DC-9, MD-80/90, and 717 jackscrew assemblies, Safety Board investigators evaluated jackscrew 

assembly overhaul procedures and practices. Specifically, investigators reviewed the DC-9 

Overhaul Maintenance Manual and visited several maintenance facilities that overhaul jackscrew 

assemblies, including Integrated Aerospace, the only contract facility currently used by Boeing 

to overhaul acme nut and screw pairs to manufacturing end play specifications. 

Safety Board investigators identified several deficiencies in the DC-9 Overhaul 

Maintenance Manual procedures and in the practices of several of the maintenance facilities that 

were visited. Specifically, the overhaul manual did not require the use of work cards 

documenting each step in the overhaul process, and (with the exception of Integrated Aerospace) 

the facilities did not use such work cards. Also, the manual called for replacement of the acme 

nut only if the end play measurement was more than 0.040 inch. Although some overhaul 

facilities had lower self-imposed end play measurement limits for replacement of the acme nut, 

at least one facility indicated that it would return an overhauled jackscrew assembly to a 

customer as long as the end play measurement did not exceed 0.040 inch. Further, the manual 

contained no requirement to record or inform the customer of the end play measurement of an 

overhauled jackscrew assembly. This means that a jackscrew assembly with an end play 

measurement of up to 0.039 inch could be represented as “overhauled” and returned to service by 

an operator that might not be aware of the high end play measurement.22 A jackscrew assembly 

could require overhaul for reasons other than excessive end play; however, it would be 

reasonable for a customer to expect that an assembly would be returned after an overhaul with an 

end play close to manufacturing specifications. 

Further, the required steps for properly conducting the end play check procedure were not 

well described and the required equipment was not specified in the DC-9 Overhaul Maintenance 

Manual. During visits to maintenance facilities, Safety Board investigators learned that the 

facilities used different methods and various tools for measuring end play. (Although 

investigators saw no evidence that the differences affected the accuracy of the results, the lack of 

standardization nonetheless increases the potential for error to occur.) The overhaul manual also 

did not contain detailed instructions on how to apply grease to the jackscrew assembly at the 

completion of the overhaul nor did it clearly specify which type of grease to use. Further, 

 

 22 Although the DC-9 Overhaul Maintenance Manual calls for more frequent end play checks (every 

1,000 flight hours) to be conducted on an overhauled unit that was reinstalled with an end play measurement 

between 0.034 to 0.039 inch, this provision is of limited value because there is no requirement for operators to be 

informed of this elevated end play measurement. Further, operators receiving an overhauled jackscrew assembly 

would not be expected to consult the overhaul manual for special maintenance instructions. 
 16 

although the manual does require that an overhauled jackscrew assembly be lubricated before it 

can be returned to the customer, investigators learned that at least one maintenance facility 

returns overhauled assemblies without lubricating them first. 

In addition, the DC-9 Overhaul Maintenance Manual did not contain detailed instructions 

nor specify appropriate equipment for checking that the proper acme screw thread surface finish 

had been applied. Many of the maintenance facilities visited by Safety Board investigators 

indicated that they relied on subvendors to ensure that the proper acme screw thread surface 

finish was applied and had no standard method for verifying that this action had occurred. 

Finally, the DC-9 Overhaul Maintenance Manual did not clearly specify appropriate 

packaging procedures for transporting jackscrew assemblies after overhaul. Although the 

overhaul manual contained detailed protective packaging instructions for jackscrew assemblies 

going into “storage,” it did not specify any such protective packaging for jackscrew assemblies 

being transported. However, it would be prudent to use protective packaging for all overhauled 

jackscrew assemblies being returned to a customer because a maintenance facility cannot be 

expected to know what the customer intends to do with an assembly after it is returned. 

Integrated Aerospace, the only maintenance facility authorized by Boeing to overhaul the 

acme nut and screw to an “as new” end play condition that meets manufacturing specifications, 

uses more rigorous and reliable overhaul procedures and significant quality control measures that 

are not used or required by the other facilities that overhaul jackscrew assemblies. For example, 

Integrated Aerospace uses detailed work cards to document each step of the overhaul process. 

Whenever Integrated Aerospace receives a jackscrew assembly with an end play measurement 

greater than 0.015 inch, it will install a new acme nut, thereby restoring the assembly to the 

manufacturing specifications for a new jackscrew assembly (0.003 to 0.010 inch). In doing so, 

Integrated Aerospace must comply with detailed specifications provided by Boeing in a service 

rework drawing, which are not contained in the DC-9 Overhaul Maintenance Manual. 

In addition, the investigation revealed that no special authorization beyond a class 1 

accessory rating is required for a maintenance facility to overhaul jackscrew assemblies in 

accordance with the DC-9 Overhaul Maintenance Manual. A class 1 accessory rating allows a 

facility to perform maintenance and alteration of a number of mechanical accessories. The 

maintenance facility is not required to demonstrate that it has the necessary capability and 

equipment to perform jackscrew assembly overhauls. Safety Board investigators found that the 

PMI of an FAA-certified maintenance facility may not even be aware that the facility is 

performing overhauls of jackscrew assemblies. 

The Safety Board concludes that deficiencies in the overhaul process increase the 

likelihood that jackscrew assemblies may be improperly overhauled. The Safety Board further 

concludes that the absence of a requirement to record or inform customers of the end play 

measurement of an overhauled jackscrew assembly could result in an operator unknowingly 

returning a jackscrew assembly to service with a higher-than-expected end play measurement. 

Therefore, the Safety Board believes that the FAA should require that maintenance facilities that 

overhaul jackscrew assemblies record and inform customers of an overhauled jackscrew 

assembly’s end play measurement. 

In addition to recording the end play measurement information provided by a 

maintenance facility when it returns an overhauled jackscrew assembly, it would also be prudent 

for operators to record end play measurements for the same assembly after it is installed on an 

airplane. The Safety Board notes that end play measurements recorded by maintenance facilities 

are likely to be obtained during bench checks. However, after the overhauled assembly is 

re-installed on an airplane, end play measurements will likely be obtained through use of the 

on-wing end play check procedure, which may yield a slightly different measurement because of 
 17 

differences in the procedure. A wear rate that is calculated using a bench-check measurement at 

one point in time compared with an on-wing measurement at a later point in time will not be as 

informative or useful as a wear rate that is calculated using measurements obtained through use 

of the same end play check method. Therefore, the Safety Board concludes that operators will 

maximize the usefulness of end play measurements and wear rate calculations by recording 

on-wing end play measurements whenever a jackscrew assembly is replaced on an airplane. 

Accordingly, the Safety Board believes that the FAA should require operators to measure and 

record the on-wing end play measurement whenever a jackscrew assembly is replaced. 

Finally, the Safety Board concludes that, because the jackscrew assembly is an integral 

and essential part of the horizontal stabilizer trim system, a critical flight system, it is important 

to ensure that maintenance facilities authorized to overhaul these assemblies possess the proper 

qualifications, equipment, and documentation. Therefore, the Safety Board believes that the FAA 

should require that maintenance facilities that overhaul DC-9, MD-80/90, and 717 series 

airplanes’ jackscrew assemblies obtain specific authorization to perform such overhauls, 

predicated on demonstrating that they possess the necessary capability, documentation, and 

equipment for the task and that they have procedures in place to (1) perform and document the 

detailed steps that must be followed to properly accomplish the end play check procedure and 

lubrication of the jackscrew assembly, including specification of appropriate tools and grease 

types; (2) perform and document the appropriate steps for verifying that the proper acme screw 

thread surface finish has been applied; and (3) ensure that appropriate packaging procedures are 

followed for all overhauled jackscrew assemblies, regardless of whether the assembly has been 

designated for storage or shipping. 

Horizontal Stabilizer Trim System Design and Certification Issues 

Acme Nut Thread Loss as a Catastrophic Single-Point Failure Mode

The DC-9 horizontal stabilizer trim system (which was also incorporated in the 

MD-80/90 and 717 series airplanes) is a critical flight system because certain failures of the 

system can be catastrophic. One such failure is the loss of acme screw and nut thread 

engagement. However, the designers of the system assumed that at least one set of the jackscrew 

assembly’s acme screw and nut threads would always be intact and engaged to act as a load path. 

Therefore, the repercussions of stripped acme nut threads and the corresponding effect on the 

airplane (including the possibility of the acme screw disengaging from the acme nut) were not 

considered in the design of the horizontal stabilizer trim system. 

At the Safety Board’s public hearing, Boeing engineers stated that they considered loss of 

the acme nut threads to be a “multiple failure event” and that such loss caused by excessive wear 

was not considered a “reasonably probable single failure” for certification purposes.23 The 

Boeing engineers indicated that the jackscrew assembly was designed to accommodate thread 

wear but acknowledged that monitoring and managing thread wear was essential to maintaining 

the integrity of the design. Similarly, an FAA certification engineer testified that thread wear 

“was not considered as a mode of failure for either a systems safety analysis or for structural 

considerations” and that the “design of the acme nut and screw provided enough over-strength so 

that regulatory requirements could be met with a significant amount of wear.” However, as the 

 

 23 The certification basis for the DC-9, MD-80/90, and 717 horizontal stabilizer trim systems was Civil 

Aeronautics Regulations (CAR) 4b. CAR 4b.320, “Control Systems,” stated that “an adjustable stabilizer shall 

incorporate means to permit, after the occurrence of any reasonably probable single failure of the actuating system, 

such adjustment as would be necessary for continued safety of the flight.” (Current certification regulations specify, 

in 14 CFR 25.671, that the airplane must be shown to be capable of continued safe flight and landing after “any 

single failure” of the actuating system.) 
 18 

Alaska Airlines flight 261 accident demonstrates, complete loss of the acme nut threads because 

of excessive wear is possible. Further, the accident jackscrew assembly is not the only jackscrew 

assembly in which excessive acme nut thread wear has occurred. Excessive wear of acme nut 

threads has occurred on other occasions as a result of inadequate lubrication, improper acme 

screw thread surface finish, and contamination. In addition, other potential rapid wear 

mechanisms may not have yet been identified. 

The Safety Board notes that the dual-thread design of the acme screw and nut does not 

adequately protect against excessive acme nut thread wear. Although Boeing contends that the 

two thread spirals along the length of both the acme screw and nut provide structural redundancy, 

the Board notes that each set of thread spirals is always carrying loads in flight and that both sets 

of thread spirals are subject to the same wear mechanisms. Thus, although the dual-thread design 

may prevent a crack in one thread set from propagating through the other thread set, both sets of 

threads remain vulnerable to simultaneous wear failure. Therefore, the Safety Board concludes 

that the dual-thread design of the acme screw and nut does not provide redundancy with regard 

to wear. 

The FAA’s certification scheme is intended to protect against catastrophic single-point 

failure conditions. Specifically, 14 CFR 25.1309 requires that airplane systems and associated 

components be designed so that “the occurrence of any failure condition which would prevent 

the continued safe flight and landing of the airplane is extremely improbable.” Further, Advisory 

Circular (AC) 25.1309-1A, “System Design and Analysis,” defines “extremely improbable” 

failure conditions as “those so unlikely that they are not anticipated to occur during the entire 

operational life of all airplanes of one type” and “having a probability on the order of 1 x 10-9 or 

less each flight hour, based on a flight of mean duration for the airplane type.” AC 25.1309-1A 

specifies that in demonstrating compliance with 14 CFR 25.1309, “the failure of any single 

element, component, or connection during any one flight...should be assumed, regardless of its 

probability,” and “such single failures should not prevent continued safe flight and landing, or 

significantly reduce the capability of the airplane or the ability of the crew to cope with the 

resulting failure condition.” 

An FAA senior certification engineer testified at the public hearing that 14 CFR 25.130924

did not apply to the jackscrew assembly acme nut because the FAA did not consider it part of a 

system. Rather, he stated that the jackscrew assembly was a “combination structural element and 

systems element” and that each category was governed by its respective regulatory requirements. 

He indicated that those portions of the jackscrew assembly below the gearbox and trim motors 

carried primary flight loads and, therefore, were covered by regulations pertaining to structure, 

not systems. He testified that the acme nut met the applicable regulatory requirements pertaining 

to structure, specifically, CAR 4b.201(a), “Strength and Deformation,” which states that structure 

“shall be capable of supporting limit loads without suffering detrimental permanent 

deformation.”25 He stated that acme nut threads that are within manufacturing specifications far 

exceed the requirements for ultimate strength and deflection limit load and added that the FAA 

does not consider deflection of worn acme nut threads to be deformation in the context of this 

regulation. 

In addition, a Boeing structures engineering manager testified that the acme nut complied 

with certification regulations pertaining to fatigue evaluation of structure. Specifically, 

CAR 4b.270, “Fatigue Evaluation of Flight Structure,” required, for “those portions of the 

airplane’s flight structure in which fatigue may be critical,” an evaluation of either fatigue 

 

 24 The predecessor to the current 14 CFR 25.1309 was CAR 4b.606, which required that all systems “be 

designed to safeguard against hazards to the airplane in the event of their malfunctioning or failure.” 

 25 A substantially similar requirement is currently contained in 14 CFR 25.305(a). 
 19 

strength (also referred to as “safe life”) or fail safe strength (also referred to as “damage 

tolerance”).26 However, the Boeing manager testified that neither evaluation was performed 

because the acme nut was not considered “fatigue critical” because of its robust design. 

It is unclear whether the design and certification of the DC-9 (and MD-80/90 and 717) 

horizontal stabilizer trim system would have been any different if the certification requirements 

for aircraft systems, in addition to those applicable to structure, had been applied to the 

jackscrew assembly acme nut during the design phase. Boeing engineers testified at the public 

hearing that the horizontal stabilizer trim system design complied with the requirements of 

14 CFR 25.1309. However, the FAA certification engineer indicated that, even if section 25.1309 

had been applicable to the entire jackscrew assembly, acme nut thread wear would not have been 

considered in the required systems safety analysis. He explained, “if you refer to a [section 

25.1309] type safety analysis to try and determine a failure rate for a wear item, there are not, 

and there’s really no such thing as a wear-critical item, never has been. The question of wear 

being a quantifiable element so that one could do a safety-type analysis for structure—it’s not 

feasible. The data to do such an evaluation is not available. It doesn’t exist.”27 

In sum, the Safety Board is concerned that Boeing and the FAA did not account for the 

catastrophic effects of total acme nut thread loss in the design and certification of the horizontal 

stabilizer trim control system. The Board is also concerned that the certification requirements for 

aircraft systems were not considered applicable to the entire jackscrew assembly, particularly the 

acme nut. Because the loss of acme nut threads in flight most likely would result in the 

catastrophic loss of the airplane, the Board considers the acme nut to be a critical element of the 

horizontal stabilizer trim control system; therefore, it should have been covered by the 

certification philosophy and regulations applicable to all other flight control systems. The Safety 

Board concludes that the design of the DC-9, MD-80/90, and 717 horizontal stabilizer jackscrew 

assembly did not account for the loss of the acme nut threads as a catastrophic single-point 

failure mode. The Safety Board further concludes that the absence of a fail-safe mechanism to 

prevent the catastrophic effects of total acme nut thread loss contributed to the Alaska Airlines 

flight 261 accident. 

Prevention of Acme Nut Thread Loss Through Maintenance and Inspection

Currently, prevention of acme nut thread loss is dependent on regular application of 

lubrication and on recurrent inspections of the jackscrew assembly to monitor acme nut thread 

wear. However, this maintenance-based approach to maintaining the horizontal stabilizer trim 

system’s structural integrity has weaknesses. 

First, current lubrication and end play check intervals may not be adequate, and their 

length can change. Research and testing relating to lubrication effectiveness and the prior service 

history of the MD-80 fleet suggests that the current 650-flight-hour lubrication interval is 

probably adequate to ensure proper lubrication of the jackscrew assembly. However, because of 

the potential for additional undiscovered rapid wear mechanisms and uncertainty regarding the 

maximum possible wear rate even for known wear mechanisms, there is no such basis for 

assuming that the current 2,000-flight-hour end play check interval is sufficient to ensure 

fleetwide safety. Further, there is no guarantee that these intervals will not eventually be 

extended, which would further reduce the level of safety. 

 

 26 Substantially similar requirements are currently contained in 14 CFR 25.571. 

 27 The Boeing manager confirmed that “the condition of wear or wear-out was not included in the original 

DC-9 fault analysis.” 
 20 

Second, and more importantly, all maintenance and inspection tasks are subject to human 

error. This investigation has identified several weaknesses in the lubrication and inspection 

procedures that could affect their intended results and compromise safety. Further, several Safety 

Board accident investigations, including the Alaska Airlines flight 261 investigation, have 

demonstrated that even simple maintenance tasks are sometimes missed or inadequately 

performed and can have catastrophic results.28 Therefore, the current horizontal stabilizer trim 

system design remains vulnerable to catastrophic failure if maintenance and inspection tasks are 

not performed properly. 

Elimination of Catastrophic Effects of Acme Nut Thread Loss Through Design

The Safety Board concludes that, when a single failure could have catastrophic results 

and there is a practicable design alternative that could eliminate the catastrophic effects of the 

failure mode, it is not appropriate to rely solely on maintenance and inspection intervention to 

prevent the failure from occurring; if a practicable design alternative does not exist, a 

comprehensive systemic maintenance and inspection process is necessary. In the case of the 

horizontal stabilizer trim system, such a design would incorporate a reliable, independent means 

for eliminating, overcoming, or counteracting the catastrophic effects of acme nut thread loss. 

The Board notes that such a design change would not necessarily need to incorporate dual 

actuators, or any other form of system redundancy; the design would only need to provide a 

mechanism for preventing stripped acme nut threads from resulting in unrecoverable movement 

of the horizontal stabilizer. The Board notes that among the several design concepts listed in 

AC 25.1309-1A that can be used to avoid catastrophic failure conditions are the following: (1) 

“designed failure effect limits, including the capability to sustain damage, to limit the safety 

impact or effects of a failure”; and (2) “designed failure path to control and direct the effects of a 

failure in a way that limits its safety impact.” 

The Safety Board concludes that transport-category airplanes should be modified, if 

practicable, to ensure that horizontal stabilizer trim system failures do not preclude continued 

safe flight and landing. Therefore, the Safety Board believes that the FAA should conduct a 

systematic engineering review to (1) identify means to eliminate the catastrophic effects of total 

acme nut thread failure in the horizontal stabilizer trim system jackscrew assembly in DC-9, 

MD-80/90, and 717 series airplanes and require, if practicable, that such fail-safe mechanisms be 

incorporated in the design of all existing and future DC-9, MD-80/90, and 717 series airplanes 

and their derivatives; (2) evaluate the horizontal stabilizer trim systems of all other 

transport-category airplanes to identify any designs that have a catastrophic single-point failure 

mode and, for any such system; (3) identify means to eliminate the catastrophic effects of that 

single-point failure mode and, if practicable, require that such fail-safe mechanisms be 

 

 28 For example, following the September 11, 1991, crash of a Continental Express Embraer 120 in Eagle 

Lake, Texas, the Safety Board concluded that the airline’s maintenance inspection and quality assurance programs 

failed to detect that the upper row of screws on the leading edge of the left horizontal stabilizer had been removed 

during maintenance and had not been replaced. The partially secured left horizontal stabilizer leading edge separated 

in flight, causing a severe nose-down pitchover. The airplane broke up in flight, and all 14 people on board were 

killed. For more information, see National Transportation Safety Board. Britt Airways, Inc., d/b/a Continental 

Express Flight 2574, In-flight Structural Breakup, EMB-120RT, N33701, Eagle Lake, Texas, September 11, 1991.

NTSB/AAR-92/04. In addition, the Board’s investigation of the July 6, 1996, uncontained engine failure on a Delta 

Air Lines MD-88 in Pensacola, Florida, determined that a fluorescent inspection process used to detect fatigue 

cracks during maintenance was susceptible to error because it involved multiple cleaning, processing, and inspection 

procedures dependent on several individuals and because of a low expectation of finding a crack. Shrapnel from the 

uncontained engine failure pierced the fuselage and entered the rear cabin. Two passengers were killed, and two 

others were seriously injured. For more information, see National Transportation Safety Board. Uncontained Engine 

Failure, Delta Airlines Flight 1288, McDonnell Douglas MD-88, N927DA, Pensacola, Florida, July 6, 1996. 

NTSB/AAR-98-01.
 21 

incorporated in the design of all existing and future airplanes that are equipped with such 

horizontal stabilizer trim systems. 

Further, the Safety Board is concerned that the FAA certified a horizontal stabilizer trim 

system that had a single-point catastrophic failure mode. The Safety Board concludes that 

catastrophic single-point failure modes should be prohibited in the design of all future airplanes 

with horizontal stabilizer trim systems, regardless of whether any element of that system is 

considered structure rather than system or is otherwise considered exempt from certification 

standards for systems. Therefore, the Safety Board believes that the FAA should modify the 

certification regulations, policies, or procedures to ensure that new horizontal stabilizer trim 

control system designs are not certified if they have a single-point catastrophic failure mode, 

regardless of whether any element of that system is considered structure rather than system or is 

otherwise considered exempt from certification standards for systems. 

Consideration of Wear-Related Failures During Design and Certification 

The Alaska Airlines flight 261 investigation revealed that the FAA certification processes 

and procedures did not adequately consider and address the consequences of excessive wear in 

the context of certifying the DC-9, MD-80/90, and 717 horizontal stabilizer trim system. In light 

of this finding, the Safety Board is concerned that the consequences of excessive wear might not 

be considered in the contexts of other certifications as well. One way to ensure that such 

consequences are considered would be to include wear-related failures in the failure modes and 

effects analyses (FMEA) and fault tree analyses that are required under 14 CFR 25.1309. Boeing 

and the FAA have accepted the premise that wear cannot be considered a mode of failure in 

systems safety analyses such as FMEAs and fault trees; however, the Board notes that standards 

developed by the Society of Automotive Engineers (SAE) specify that wear should be 

considered in FMEAs.29 Design guidelines should require that a wear-related failure be assumed 

and that the results of such a failure be evaluated. 

The Safety Board concludes that the certification requirements applicable to 

transport-category airplanes should fully consider and address the consequences of failures 

resulting from wear. Therefore, the Safety Board believes that the FAA should review and revise 

aircraft certification regulations and associated guidance applicable to the certification of 

transport-category airplanes to ensure that wear-related failures are fully considered and 

addressed so that, to the maximum extent possible, they will not be catastrophic. 

 

 29 SAE’s Aerospace Recommended Practices 4761, “Guidelines and Methods for Conducting the Safety 

Assessment Process on Civil Airborne Systems and Equipment,” in paragraph G.3.2.2.1 of appendix G, lists wear 

among the failure modes to consider in performing an FMEA. 
 22 

Recommendations 

As a result of the investigation of the Alaska Airlines flight 261 accident, the National 

Transportation Safety Board makes the following recommendations to the Federal Aviation 

Administration: 

Issue a flight standards information bulletin directing air carriers to instruct pilots that in 

the event of an inoperative or malfunctioning flight control system, if the airplane is 

controllable they should complete only the applicable checklist procedures and should 

not attempt any corrective actions beyond those specified. In particular, in the event of an 

inoperative or malfunctioning horizontal stabilizer trim control system, after a final 

determination has been made in accordance with the applicable checklist that both the 

primary and alternate trim systems are inoperative, neither the primary nor the alternate 

trim motor should be activated, either by engaging the autopilot or using any other trim 

control switch or handle. Pilots should further be instructed that if checklist procedures 

are not effective, they should land at the nearest suitable airport. (A-02-36) 

Direct all certificate management offices to instruct inspectors to conduct surveillance of 

airline dispatch and maintenance control personnel to ensure that their training and 

operations directives provide appropriate dispatch support to pilots who are experiencing 

a malfunction threatening safety of flight and instruct them to refrain from suggesting 

continued flight in the interest of airline flight scheduling. (A-02-37) 

As part of the response to Safety Recommendation A-01-41, require operators of Douglas 

DC-9, McDonnell Douglas MD-80/90, and Boeing 717 series airplanes to remove 

degraded grease from the jackscrew assembly acme screw and flush degraded grease and 

particulates from the acme nut before applying fresh grease. (A-02-38) 

As part of the response to Safety Recommendation A-01-41, require operators of Douglas 

DC-9, McDonnell Douglas MD-80/90, and Boeing 717 series airplanes, in coordination 

with Boeing, to increase the size of the access panels that are used to accomplish the 

jackscrew assembly lubrication procedure. (A-02-39) 

Establish the jackscrew assembly lubrication procedure as a required inspection item that 

must have an inspector’s signoff before the task can be considered complete. (A-02-40) 

Review all existing maintenance intervals for tasks that could affect critical aircraft 

components and identify those that have been extended without adequate engineering 

justification in the form of technical data and analysis demonstrating that the extended 

interval will not present any increased risk and require modifications of those intervals to 

ensure that they (1) take into account assumptions made by the original designers, (2) are 

supported by adequate technical data and analysis, and (3) include an appropriate safety 

margin that takes into account the possibility of missed or inadequate accomplishment of 

the maintenance task. In conducting this review, the Federal Aviation Administration 

should also consider original intervals recommended or established for new aircraft 

models that are derivatives of earlier models and, if the aircraft component and the task 

are substantially the same and the recommended interval for the new model is greater 

than that recommended for the earlier model, treat such original intervals for the 

derivative model as “extended” intervals. (A-02-41) 

Conduct a systematic industrywide evaluation and issue a report on the process by which 

manufacturers recommend and airlines establish and revise maintenance task intervals 
 23 

and make changes to the process to ensure that, in the future, intervals for each task (1) 

take into account assumptions made by the original designers, (2) are supported by 

adequate technical data and analysis, and (3) include an appropriate safety margin that 

takes into account the possibility of missed or inadequate accomplishment of the 

maintenance task. (A-02-42) 

Require operators to supply the Federal Aviation Administration (FAA), before the 

implementation of any changes in maintenance tasks intervals that could affect critical 

aircraft components, technical data and analysis for each task demonstrating that none of 

the proposed changes will present any potential hazards, and obtain written approval of 

the proposed changes from the principal maintenance inspector and written concurrence 

from the appropriate FAA aircraft certification office. (A-02-43) 

Pending the incorporation of a fail-safe mechanism in the design of the Douglas DC-9, 

McDonnell Douglas MD-80/90, and Boeing 717 horizontal stabilizer jackscrew 

assembly, as recommended in Safety Recommendation A-02-49, establish an end play 

check interval that (1) accounts for the possibility of higher-than-expected wear rates and 

measurement error in estimating acme nut thread wear and (2) provides for at least two 

opportunities to detect excessive wear before a potentially catastrophic wear condition 

becomes possible. (A-02-44) 

Require operators to permanently (1) track end play measures according to airplane 

registration number and jackscrew assembly serial number, (2) calculate and record 

average wear rates for each airplane based on end play measurements and flight times, 

and (3) develop and implement a program to analyze these data to identify and determine 

the cause of excessive or unexpected wear rates, trends, or anomalies. The Federal 

Aviation Administration (FAA) should also require operators to report this information to 

the FAA for use in determining and evaluating an appropriate end play check interval. 

(A-02-45) 

Require that maintenance facilities that overhaul jackscrew assemblies record and inform 

customers of an overhauled jackscrew assembly’s end play measurement. (A-02-46) 

Require operators to measure and record the on-wing end play measurement whenever a 

jackscrew assembly is replaced. (A-02-47) 

Require that maintenance facilities that overhaul Douglas DC-9, McDonnell Douglas 

MD-80/90, and Boeing 717 series airplanes’ jackscrew assemblies obtain specific 

authorization to perform such overhauls, predicated on demonstrating that they possess 

the necessary capability, documentation, and equipment for the task and that they have 

procedures in place to (1) perform and document the detailed steps that must be followed 

to properly accomplish the end play check procedure and lubrication of the jackscrew 

assembly, including specification of appropriate tools and grease types; (2) perform and 

document the appropriate steps for verifying that the proper acme screw thread surface 

finish has been applied; and (3) ensure that appropriate packing procedures are followed 

for all returned overhauled jackscrew assemblies, regardless of whether the assembly has 

been designated for storage or shipping. (A-02-48) 

Conduct a systematic engineering review to (1) identify means to eliminate the 

catastrophic effects of total acme nut thread failure in the horizontal stabilizer trim system 

jackscrew assembly in Douglas DC-9 (DC-9), McDonnel Douglas MD-80/90 

(MD-80/90), and Boeing 717 (717) series airplanes and require, if practicable, that such 

fail-safe mechanisms be incorporated in the design of all existing and future DC-9, 

MD-80/90, and 717 series airplanes and their derivatives; (2) evaluate the horizontal 
 24 

stabilizer trim systems of all other transport-category airplanes to identify any designs 

that have a catastrophic single-point failure mode and, for any such system; (3) identify 

means to eliminate the catastrophic effects of that single-point failure mode and, if 

practicable, require that such fail-safe mechanisms be incorporated in the design of all 

existing and future airplanes that are equipped with such horizontal stabilizer trim 

systems (A-02-49) 

Modify the certification regulations, policies, or procedures to ensure that new horizontal 

stabilizer trim control system designs are not certified if they have a single-point 

catastrophic failure mode, regardless of whether any element of that system is considered 

structure rather than system or is otherwise considered exempt from certification 

standards for systems. (A-02-50) 

Review and revise aircraft certification regulations and associated guidance applicable to 

the certification of transport-category airplanes to ensure that wear-related failures are 

fully considered and addressed so that, to the maximum extent possible, they will not be 

catastrophic. (A-02-51) 

Acting Chairman CARMODY and Members HAMMERSCHMIDT, BLACK, and 

GOGLIA concurred with these recommendations. 

 By: Carol J. Carmody 

 Acting Chairman