Aircraft Accidents and Lessons Unlearned XLVIII: TWA Flight 841

On April 4, 1979, Trans World Airways (TWA) flight 841, a Boeing B727-31, registration number N840TW, suffered an uncontrolled maneuver, beginning at 39,000 feet; the aircraft rapidly descended for seventy-one seconds before it stabilized at 5,000 feet near Saginaw, Michigan. The number 7 leading edge slat departed the aircraft, the tracks and actuator for the number 7 slat were substantially damaged.

The flight left JFK Airport for Minneapolis roughly fifty-five minutes before the event began.  This meant the aircraft had been at cruise for close to an hour, meaning no secondary flight controls, e.g., four sets of flaps, eight slats, six Krueger flaps or ground spoilers would have been extended. The first indication of a problem, per the pilot interviews, was that the Captain’s altitude director indicator – ADI – showed a right bank of twenty to thirty degrees. From this indication to the recovery at 5,000 feet, the flight crew were in reaction mode.

The National Transportation Safety Board (NTSB), in report AAR-81/08, determined that the Probable Cause of the inflight upset was, “… the isolation of the number 7 leading edge slat in the fully or partially extended position after an extension of the numbers 2, 3, 6 and 7 leading edge slats and the subsequent retraction of the numbers 2, 3 and 6 slats, and the captain’s untimely flight control inputs to counter the roll resulting from the slat asymmetry. After eliminating all probable individual or combined mechanical failures or malfunctions which could lead to slat extension, the Safety Board determined that the extension of the slats was the result of the flight crew’s manipulation of the flap/slat controls.” Report AAR-81/08 went on to say, “Contributing to the captain’s untimely use of the flight controls was distraction due probably to his efforts to rectify the source of the control problem.”

The NTSB investigators’ statements bordered on accusatory – without evidence – that the flight crew introduced the slat extension by their, “… manipulation of the flap/slat controls.” Unless the flight crew stated that they extended these secondary flight controls, no one should have assumed that any slats were extended at cruise. However, NTSB investigators held that the flight crew either deliberately or unintentionally extended flight controls due to the erasure of all but nine minutes of the cockpit voice recorder (CVR) recording. Report AAR-81/08 called this action ‘a distraction’; he said, “… the Captain’s untimely use of the flight controls was a distraction due probably to his efforts …” Use of the word ‘probably’ was not in relation to probable cause; it was used in lieu of fact. The ‘probably’ was not a definitive statement; it was not analysis. It demonstrated that NSTB report AAR-81/08, while supposedly based on Factual information, had been founded on an allegation.

On the B727, the CVR erase button is on the second officer (SO) station’s side panel. The allegation meant that, not only were the Captain’s actions suspect, but that the two copilots: the first officer (FO) and the SO, would have been complicit in the ‘distraction’, otherwise the Captain would have had to set the brakes, get out of his seat and hit the ERASE button, if the SO refused. The allegation would have also assumed that CVR transcripts were indisputable, which gets to the heart of many accident report confusions, that CVRs were 100% reliable for determining accident cause. The CVR is, and was, never 100% reliable. CVR recording quality has always been compromised by any and all cockpit noises, e.g., aural alerts, wind noise, conversation overlaps, etc. If, in this case, the pilots did erase the CVR, was this an acceptable action? Questionable. However, after reading the NTSB’s reaction in AAR-81/08, it would have been understandable why any pilot would remove ambiguous CVR information. In AAR-81/08, the NTSB investigator assumed – without fact – misconduct on the Captain’s part. Furthermore, NTSB Board Member Francis H. McAdams agreed with this line of thought. He wrote a dissent that disagreed with the Board’s findings and questioned how they interviewed the Captain. It was important to note the four other Board Members did not question the Findings. What did this say about how the other four political appointees grasped the nature of the allegation?

AAR-81/08 was, as are all accident reports, vital to industry’s understanding of what happened, to prevent reoccurrence. AAR-81/08’s Probable Cause never addressed the Root Cause. The pilot’s actions, during the event were responsive to the probable cause: the uncontrolled maneuver. However, the Root Cause should have answered the question: Why did the number 7 slat come out of the stowed position in cruise?

On page 18 of the report, titled: History of B-727 Leading Edge Slat Problems, the report stated, “According to FAA service difficulty reports (SDRs), from the beginning of 1970 through the end of 1973, seven cases of a single leading edge slat extension and separation on B-727’s during flight were reported without mention of whether the extensions were scheduled or unscheduled.” Pause here to direct attention to the use of the words ‘scheduled and unscheduled’. Fifty-seven minutes into cruise, the wings were ‘clean’; lift-augmenting flight controls, i.e., the slats, had been stowed for nearly an hour; there would be no reason to extend them until Approach. It would be dangerous and counter to design.

The Probable Cause stated, in part, “… after an extension of the numbers 2, 3, 6 and 7 leading edge slats and the subsequent retraction of the numbers 2, 3 and 6 slats …” Aside from the unusual insinuation of extending slats in cruise, the report suggested that the crew isolated one lone slat – number 7. This suggestion demonstrated that investigators had a fundamental misunderstanding of the B727 slats’ operation. No slat on the B727 operated independently, therefore, there could be no ‘scheduled’ extension of the number 7 slat – the flight crew could NOT do that. Slats 2, 3, 6 and 7 are deployed together when the flap handle is placed in the Flaps – Two Degrees detent. Slats 1, 4, 5, and 8 plus all six Krueger leading edge flaps all deploy when the flap handle is placed in the Flaps – Five Degrees detent and beyond to forty degrees. The problem with AAR-81/08 was that investigators kept referring back to a single slat deployment, which the pilots … could … not … do; even activating the flaps manually using the electrical system, the crew could not – could not – deploy a single slat, by itself. The number 7 slat deployment was unintentional and unexpected. The report went on to say, “… we recognize that if the No. 7 slat did not extend as the consequence of some series of failures and malfunctions in the slat system, then it must have been extended as a result of flightcrew action.”

Did NTSB investigators ask if a slat deployment would have triggered a warning? Actuation of any slat would have resulted in an illumination of the leading-edge device (LED) deploy/unsafe light indicator on the pilots’ instrument panel. Each of the B727’s eight slat LED switches is internal, meaning the switch is inside the actuator. The LED indicator light is Extinguished when the slat is stowed; Yellow when the slat is in transit and Green when the slat is deployed. This fact should have led investigators to ask the pilots: “Did you see an LED indication prior to the event?”

Consider that with all the LEDs retracted in cruise, for the captain to extend slats, he would have had to reach over the throttle quadrant and pulled the flap handle out of its ZERO detent – or – actuated the manual electric controls over his head. An out-of-configuration alarm, e.g., a takeoff aural warning would have sounded. The FO and/or SO would have questioned the action. Even if the Captain recklessly did all this, the FO and/or SO would have reported the incident to the Chief Pilot, who, with the Federal Aviation Administration Principal Operations Inspector, would have acted.

Did the NTSB investigators conduct a thorough maintenance investigation? The number 7 slat was missing, its slat tracks and actuator were severely damaged. The right wing displayed impact scratches and the righthand outboard aileron had been hit by the departing slat. The NTSB performed some basic inquiries into N840TW’s maintenance history and the slat system’s inspection and maintenance, but it was not deep or broad enough; the past maintenance examination was cursory. As the probable cause stated, “… if the No. 7 slat did not extend as the consequence of some series of failures and malfunctions in the slat system, then it must have been extended as a result of flightcrew action.” This meant that the investigators made ineffective explorations into why the number 7 slat was out of configuration.

In addition, the NTSB depended too heavily on Boeing to analyze their own aircraft. This raises the question: Should any manufacturer be expected to fairly review its products, especially when the determination could devastate said manufacturer? This cast doubt, not just upon the NTSB’s AAR-81/08 report’s quality, but also upon Boeing’s ability to be forthcoming about their airplane’s safety. It would have also allowed Boeing to fix any problems without any consequences.

One last item suggests the investigators’ unfamiliarity with the B727 slat system. On page 25, where the investigators justified faulting the Captain, the report stated, “… the air loads on the slat would have subjected the slat actuator rod to a compressive load of about 700 pounds and about 9 percent less if the outboard aileron was floating.” It was unclear what the investigators meant by ‘floating’, but the problem with this scenario is that the outboard ailerons are locked out at cruise – they do not move; they do not unlock until five degrees of flaps are selected – the numbers 2, 3, 6 and 7 slats extend at two degrees of flaps. In addition, the slats do not unlock the outboard ailerons, the flaps do; the mechanical device that unlocks the outboard aileron comes off the flap transmission, not the slat actuators. Therefore, the investigators should have tested, not only all eight slats but the four flap systems as well. If the slats were extended intentionally, the flaps would have moved as well.

The aviation community needs to trust the investigatory process, be convinced it is legitimate. The NTSB must not just expect quality and impartiality from the five transportation mode agencies it investigates, it must rise to the same standards itself. We will never know the Root Cause for what happened to TWA flight 841 on April 4, 1979; it is small consolation that the B727 has limited presence today in the United States, if at all. But the B727 still flies; other Boeing products have similar, if not identical, slat systems to the B727. If only the Root Cause had been found.