James Reason and the Swiss Cheese model - AW&ST

Skyhawk Prang: Lining Up Swiss Cheese Holes

Dec 5, 2016by John Croft in Things With Wings

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Checklists, procedures and aircraft design are generally pretty good at helping pilots catch errors that could lead to an accident. But when a California pilot added a backwards control lock and an iPad into the mix, the proverbial holes in the Swiss cheese model of accident causation lined up and he ended up with a broken Cessna Skyhawk.

The Swiss cheese model is based on the idea that safety nets are like slices of Swiss cheese with randomly placed holes (representing the inherent flaws in any safety system). Accidents and incidents occur when the holes in the stack of slices line up and allow an error or a series of errors to develop into what you see in the picture below. 

This relatively generic fender-bender got the interest of the NTSB, which sent the pilot three sets of questions to expand on the information he provided in the standard government form used to report civil and public aircraft accidents and incidents. 

Why would the NTSB be so curious? Maybe because the accident occurred despite having so many slices (safety nets), meaning there could be lessons learned for the larger general aviation population. I count holes in at least six slices that had to line up, including a yoke-mounted iPad (a device put in the aircraft to boost safety) with an oddball orientation.

The 500-hr. private pilot flew himself and three passengers had departed the home airport, where the C172 is usually hangared, and stopped at a destination airport for lunch on October 19. 

It was there that the pilot inserted the Cessna’s coat hanger-like control lock to prevent any wind gusts from damaging the controls ailerons and elevator. Unfortunately, he installed it backwards (which he later said was in part because he rarely used the control lock since the C172 was hangared).  

Installed correctly, a metal plate would block the pilot from installing the keys in the ignition until the control lock is removed. Hole 1. 

Back at the aircraft after lunch, the holes in more safety net slices started to line up.

-    The pre-flight walk-around inspection calls for three separate aileron and elevator control movement checks that would have revealed that the control lock was installed (see checklist below). The pilot skipped all three. (Holes 2, 3, 4)

-    During the pre-takeoff check in the run-up area, the pilot noticed that the fuel gauges didn’t look quite right, so he changed his normal routine, shutting down the engine to visibly check the tanks. When he returned, he finished the checklist with the exception of the line that reads: “Controls Free and Correct”. (Hole 5)

-    The pilot was also using an iPad mini with a yoke mount, and he had positioned the iPad so that it was relatively flat (facing the ceiling), which the NTSB said blocked his view of the installed control lock (Hole 6).

While the odds of the holes in six slices Swiss cheese are probably fairly small, the incident did have a relatively happy ending in that the pilot realized the aircraft was not responding correctly.

“I did not know what the problem was,” he told the NTSB, “but knew I needed to abort the takeoff and stop the aircraft.”

From the NTSB's viewpoint in the factual report (click here to see the report), here's how it ended. 

"The airplane lifted off about half-way down the 3,600 foot runway, but when it was at an altitude of about 20 feet above the ground, it stopped climbing," reads the report. "The pilot 'immediately recognized something was wrong,' aborted the departure, and the airplane landed on the remaining runway. The pilot was unable to stop the airplane on the runway, and it sustained substantial damage to the fuselage as a result."

While this incident was not severe, too often the results of a forgotten control locks are disastrous, including the aborted takeoff overrun of a Gulfstream G-IV at the Laurence G. Hanscom Field in Massachusetts in May 2014 that left all seven people on board dead. The pilots missed many of the same standard checklist procedures as this Skyhawk pilot.