Crunch Time

Aviation Machinist Mate 3rd Class Brent Laube conducts a low power turn of an F/A-18F Super Hornet under the supervision of plane captain Aircraft Structural Mechanic 3rd Class Dale Little. (Photo by Aviation Electronics Technician 1st Class James Carter)

Aviation Machinist Mate 3rd Class Brent Laube conducts a low power turn of an F/A-18F Super Hornet under the supervision of plane captain Aircraft Structural Mechanic 3rd Class Dale Little. (Photo by Aviation Electronics Technician 1st Class James Carter)


As many readers of Mech already know, some of the most common ground mishaps plaguing the FA-18 community are crunches involving doors 64L and 64R. For non-Hornet maintainers, those are the two engine bay doors that clamshell open on the keel of the aircraft. The reason they keep getting crunched is because when they are open they are directly in the path of the trailing edge flaps (TEF). The vast majority of these crunches occur during non-pilot maintenance turns.

As with most mishaps, there is no one specific reason that these doors get crunched. Two previous editions of Mech featured articles about these specific crunches, each of which had different causes. The article “Best Practice: The Turn,” featured in the Winter 2013-14 Mech was caused by intentionally moving the flaps with door 64L open, while the article “How Did We Get Here,” published in Summer 2013 had a crunch involving an unintentional flap movement with the door open. A web enabled safety system (WESS) review of SIRs and HAZREPs involving these doors show a mixture of crunches caused by intentionally moving the flaps with the doors open and instances where the flaps moved unintentionally with the doors open. Our most recent crunch was one of the latter.

While the author was not personally involved in this turn, he was one of the investigators tasked with figuring out what happened after the fact. In many of these situations, the cause is relatively cut-and-dry. This case was certainly not going to any quick answer.

The mishap started out as an out-of-phase low power turn along with engine installation leak checks. The turn operator was a previously qualified turn operator working on his recertification under the instruction of a department head squadron pilot. The event was properly briefed and the pre-turn walk around was standard with all the aircraft doors closed during the first engine start.

There were a couple of issues with the right engine start and once the engine finally did start the right generator failed to come online. The turn operator and turn instructor then decided to start the left engine, knowing that the left generator could power the entire electrical system of the aircraft. Once the electrical system came online, the turn operator and turn instructor noted that the aircraft had both R GEN and R DC FAIL cautions, indicating that both the AC and DC systems were inoperative on the right engine. Approximately 45 minutes into the turn, the left engine was secured to conduct leak checks. The intent was to execute a crossbleed start of the left engine once leak checks were completed. At this point the TEFs were full down and doors 64L and 64R were both being held opened by maintainers. Approximately two minutes later, the flaps began to close with the flap switch still in full. While one maintainer was quick to close his door before the flaps closed, the other was not as quick and door 64R was impacted by the right TEF, bending the door and causing damage.

Figuring out why the flaps moved unintentionally was a thorough lesson in FA-18F electrical and hydraulic system redundancy. When the turn operator secured the left engine, with the only operable generator, he induced a dual generator failure. Normally, the flight control computers (FCC) and the essential bus would be powered indefinitely by the permanent magnet generators (PMG) on the operating engine. In this case, the right generator and PMG were both failed (R GEN and R DC FAIL cautions), which left the battery as the only electrical source powering the essential bus.

In order to save battery power, the battery switch is mechanized to shut off two minutes after AC power is secured on deck. Once the battery was secured, the FCCs were no longer powered, which left the aircraft with hydraulic power to the flight control surfaces but no inputs from the FCCs. In this configuration, the TEFs will drive up from 30 degrees down to five degrees down. In this case, they did so with doors 64L and 64R in the way.

This is a rather complicated scenario. Based on the turn operator PQS, it is impractical to expect a turn operator to have this extensive depth of system knowledge. It is also impractical to expect the turn instructor to have that level of systems knowledge, despite being a senior squadron pilot, since aircrew in the Super Hornet don’t spend much time studying scenarios involving unpowered FCCs, a situation that would lead to a loss of control and ultimately to an ejection airborne. Now, even though this is a complicated scenario which is not likely to be repeated often, there are good lessons that can be learned.

In our squadron, as in most other squadrons, we preach “by the book” maintenance. In this case “the book” refers to the interactive electronic technical manual (IETMS), which lists several cautions related to opening doors 64L and 64R while the engines are turning. If the flaps are up, then IETMS directs securing hydraulic power with the hydraulic system 1A and 2B manual shutoff valves in door 51L and 51R and installing TEF support brackets to keep the TEFs from sagging once hydraulic power is removed. If the TEFs are down, like in this example, then IETMS cautions “To prevent damage to trailing edge flap or door 64L/R if door is open, hydraulic system 1A and 2B manual shutoff valve in door 51L and 51R must be positioned to OFF to prevent actuation of trailing edge flaps.” This seems rather straightforward; however, closer investigation revealed that these shutoff valves are almost never used. Part of the reason is because these types of “all shops” turns usually involve FCS IBITs, which can’t be conducted with the flaps disconnected. Additionally there seems to be a general distrust of these valves by maintenance personnel. My squadron was only able to gather anecdotal evidence, but most maintenance personnel interviewed stated that these valves are never used because they don’t work.

This author was unable to find any SIRs or HAZREPs where the shutoff valves were used and failed, but the sheer number of crunches would support that these valves are not used as often as they are called for in IETMS. If the shutoff valves had been used in this scenario, they would have isolated the TEFs from the hydraulic system and prevented their retraction into door 64R. Had the valves been used and failed then NAVAIR could conduct an engineering investigation (EI) into why the valves failed and develop a fix. Many squadrons have also developed other techniques to prevent crunches, such as having maintainers hold the doors instead of propping them open, but our mishap proved that technique doesn’t always prevent a mishap.

In the big picture, there are many situations both known and unknown that cause the flaps to move in the FA-18. Every time door 64L or 64R are open with the hydraulic system powered there is a risk of the flaps moving and causing a mishap. If safety measures are being ignored fleetwide then that is a systemic problem that needs to be fixed. If the hydraulic isolation valves work, then a “by-the-book” maintenance department should be using them whenever IETMS calls for them.

If they don’t work, then the community needs to document cases of failure so a proper solution can be developed. VFA-213 has reviewed our maintenance procedures and implemented controls to ensure that the hydraulic shut-off valves and TEF locks are used whenever called for by IETMS. Since safety measures are already in place to prevent this problem, the best thing to do is double check our steps to eliminate damage to our assets.