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The Pilot<\/h2>\n
The final report from the NTSB doesn\u2019t tell us much about the pilot. He was a 60-year-old male with a first class medical. He held an ATP certificate and was a CFI\/MEI. The NTSB listed him as an \u201coccupational pilot.\u201d<\/p>\n
But an online obituary gives us more detail. He received a discovery flight as a gift for high school graduation and fell in love with flying. He went to Embry-Riddle Aeronautical University, and then worked as a freight dog, followed by a 30-year career with US Air and American Airlines. He capped off his career as an A321 captain, with over 35,000 hours in his logbooks.<\/p>\n
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The NTSB<\/h2>\n
In their inspection of the shattered and charred wreckage, the NTSB investigators lucked out, as the tail section survived relatively intact, allowing the discovery that both elevator trim tabs had been installed upside down and \u201creversed.\u201d<\/p>\n
The right-hand trim tab was installed on the left elevator and vice versa.<\/p>\n
The NTSB noted that this \u201cresulted in a reversal of the direction of the trim tabs\u2019 movements; a command of nose-up trim from the cockpit controls would incorrectly move the trim tabs in the nose-down direction, and vice versa.\u201d<\/p>\n
The NTSB believed this error happened at the recent annual, listing the probable cause of the accident as \u201cthe mechanic\u2019s inadvertent installation of the elevator trim tabs in reverse, which resulted in the pitch trim system operating opposite of the pilot\u2019s input and the pilot\u2019s subsequent loss of control.\u201d<\/p>\n
So how could such a maintenance error have occurred? And why was it missed by everyone?<\/p>\n
Analysis & Discussion<\/h2>\n
In the course of the twin\u2019s four-month, $42,951.65 annual inspection, corrosion was discovered on all of the flight control surfaces. They were removed, repaired, repainted, and re-installed \u2014 including, of course, the trim tabs. They were part of Discrepancy #38 on the 13-page work order, with a logbook entry stating that the work was done in accordance with the Piper maintenance manual.<\/p>\n
<\/center>But the NTSB seems to cast doubt on the use of the maintenance manual, as they included both the parts catalog and the service manual in its report.<\/p>\n
Interestingly, the parts catalog actually shows the elevator trim tab control horn in the wrong place, which could lead to upside-down installation, but the NTSB calls attention to the fact that the preface matter of the catalog clearly states that the book shouldn\u2019t be used for maintenance, and that the control horns are properly illustrated in the service manual.<\/p>\n
But oddly, the summary of their interview of the director of maintenance (DOM) for the Part 43 shop never mentions the parts catalog. So why was it included? Did one of the employees say something off the record?<\/p>\n
And while the DOM emphasized to the NTSB that he \u201ctries to drill it into the guys\u201d that they always need to reference the maintenance manuals, that wasn\u2019t so easy for the mechanics to do. The maintenance manuals were on computers that, at the time of the twin\u2019s annual, were not located in the maintenance hangar, requiring mechanics to leave the job, go to a computer in a nearby office, check whatever info they needed, then go back to the hangar.<\/p>\n
<\/center>Why?<\/p>\n
Apparently, at one time, the shop had computers in the hangar, but \u201cthe mechanics weren\u2019t getting as much work done\u201d as before they had the computers, so the management decided the computers were \u201ca distraction\u201d and moved them to the office space.<\/p>\n
An additional potential confounder noted by the NTSB was that the shop also maintained a non-pressurized version of the Navajo, and that this version features a bottom-mounted control horn on the single elevator trim tab, unlike the pressurized version, which has the control horns located on the tops of the twin elevator trim tabs.<\/p>\n
The IA on point for the annual stated that he examined the primary flight controls for proper movement after they were reinstalled, but admitted that he did not verify proper movement of the elevator trim tabs, so the shop missed the fact that they had been re-installed incorrectly.<\/p>\n
Now, elevator trim tabs are like ants: Crazy powerful for their size. If you don\u2019t believe that, I urge you to fly to a safe altitude, set up for cruise flight, then \u2014 hands off \u2014 rapidly deploy your elevator trim tab full forward and see what happens. After you recover, and let your stomach settle back down, do the same thing with full aft trim.<\/p>\n
But could improperly rigged elevator trim tabs really have caused the loss of control? Well, as noted, trim tabs are powerful. But unlike say, improperly rigged ailerons undiscovered until liftoff \u2014 where you have mere seconds to sort out what is happening \u2014 this is a slightly slower motion emergency, and trim tabs can generally be overpowered. (Disclaimer: I\u2019ve never flown a Navajo, and the Navajo\u2019s trim tabs look pretty beefy, making up a larger percentage of the elevator\u2019s surface area than airplanes I\u2019m more familiar with so Navajo fliers, please chime in on this.)<\/p>\n Shedding more light on the loss of control from the tabs, however, is the FAA inspector\u2019s statement. He talked to family members at the crash site the day after the accident. The pilot\u2019s son reportedly told the inspector that the airplane \u201ctook both hands and full nose up trim\u201d to get \u201coff the ground,\u201d and that this had been true \u201cever since they first got the airplane.\u201d<\/p>\n This suggests that the pilot was in the habit of setting full up trim and hauling back. But with the tabs reversed, the plane is now trying to nose down, even while he\u2019s got the yoke waaaaay back. It doesn\u2019t take much imagination to picture the wild see-saw ride that ensued.<\/p>\n Additionally, with the plane fighting him to dive, you can see why attempting to trim the nose \u201cdown\u201d (which in this case would have raised it) might have seemed a suicidal choice to the pilot \u2014 rather than a logical trouble-shooting step.<\/p>\n The mistake with the trim tabs is mechanical malpractice, but what about the pilot? Could he have \u2014 should he have \u2014 caught the mistake?<\/p>\n Well, in case you don\u2019t know, post-maintenance is the time to preflight like your life depends on it \u2014 because it does.<\/p>\n And while preflighting trim can be difficult, especially unassisted, it\u2019s hardly impossible. You can deploy it fully one direction, get out of the plane, verify its position, get back in the plane, deploy it fully the other direction and once again, clamber out and check where it is.<\/p>\n Should the pilot have done that? Given that he must have known the tail feathers were removed, worked on, and re-installed: Yes. Absolutely. And for all we know, he did.<\/p>\n Because for that \u201cinspection\u201d to work, he would have needed to understand how the trim tab should look for a given setting, and frankly, a lot of pilots don\u2019t. There\u2019s no shame in this. External inspection of trim tab deployment angles just isn\u2019t one of those things we deal with during routine operations \u2014 I don\u2019t think I\u2019ve ever seen a trim tab position check on a preflight checklist.<\/p>\n So a good takeaway for all of us is to review this smallest of secondary controls. A properly-installed and rigged elevator trim tab points in the opposite direction from the commanded setting. Dialing full nose-up trim in the cockpit will result in a downward-angled trim tab, and vice versa.<\/p>\n For my student pilot readers, this is because trim tabs move control surfaces in the opposite direction. Sticking with the nose-up command example, the down-angled trim tab forces the elevator upward in response. The up elevator, in turn, forces the tail of the airplane downward, which, in turn, raises the nose.<\/p>\n Is it any wonder folks sometimes get confused staring at the trim tab at the back of the airplane, trying to visualize all the various motions and forces at play?<\/p>\n And, finally, there\u2019s one more takeaway to consider from this accident: Could that long airline career have worked against the pilot, rather than giving him the increased wisdom we usually expect with increased experience?<\/p>\n He spent decades in an environment in which maintainers were specialists in a limited number of airframes and the airplanes were sufficiently complicated that \u2014 while flight crews still did walk arounds \u2014 there\u2019s a lot they simply couldn\u2019t see, so in airline operations, the needle on the trust\/verify meter shifts heavily toward trust.<\/p>\n But GA is different. Our maintenance folks work on a lot of different airplanes, and in many cases the pilots know the systems better than the mechanics. Returning-to-GA retired airline folks should take note of this takeaway: In GA, you need to move the trust\/verify needle closer to verify.<\/p>\n![\"\"](\"https:\/\/generalaviationnews.com\/wp-content\/uploads\/2023\/05\/Human-factors-Trim-tabs.jpg\")
The Takeaway<\/h2>\n