When the airplane has the measles

Every pilot knows (or should know) that when something on an airplane doesn’t work, it needs to be either repaired or labeled as inoperative. Even in the most basic of general aviation airplanes, certain items must be on board and working in order for the FAA to consider the aircraft airworthy. With larger airplanes, such as business aircraft and airliners, these parts are often divvied up into two broad categories: the minimum equipment list (MEL) and the configuration deviation list, or CDL. Both categories can be further broken down, and are organized in an industry-wide, standardized fashion. This makes it much easier for aircraft manufacturers, owners, operators, mechanics, the FAA, and squadrons of lawyers to speak the same language. It also makes buying and selling airplanes and parts easier to track and understand.

Broadly speaking, the MEL is a list of all items that are installed on an aircraft, or that can be, and what their operational status must be for the airplane to legally fly. Instead of just grounding a multi-million dollar jet scheduled to carry hundreds of people because one of the bulbs in the landing lights doesn’t work, the MEL is written in such a way that the operator can delay a repair for a certain amount of time. This is called a deferral, and deferrals, like the MEL itself, are categorized. Must-have items have short windows in which a repair is needed. That window might be seven to ten days, or it might be measured within so many flights (“cycles”). Less critical items, such as certain dents, scratches, or torn carpet, may be deferred for months or even years. In this case, the operator simply needs to track the item and continue to monitor it to make sure that it doesn’t deteriorate or go beyond the standards established by the manufacturer.

The CDL works the same way, but it has to do with items that are attached to the airplane or that can be removed or that can fall off. Static wicks fall under this category. So too do such items as landing gear doors that might be removed for a repair to the door. CDL items usually come with a performance penalty of some sort because their absence can increase drag.

Pilots are notified of MEL and CDL items two ways. The first is on the dispatch release that is issued for each flight. Every MEL that can affect the operation or the performance of the airplane is listed on the release. The second method is by a sticker in the cockpit. The sticker is placed on or near the panel or switch that is affected, so the notification of that burned-out light bulb would be placed on the panel next to the landing lights. CDL stickers are usually placed in a location where the FAA has stipulated such stickers will go, with the requirement being that they be easily seen and identifiable. On the sticker will be information such as the MEL number, the logbook page number, and whatever else that carrier and its FAA oversight office deem necessary.

Once in a while, an airplane comes along that has a bunch of stickers. In my case, my airline used little red circle stickers, like garage sale price tags, to identify deferred items. When a bunch showed up, we said the airplane had “the measles.” Frequently, several stickers meant that one deferral had affected multiple aircraft systems, but not always. I recently had an airplane in my current job that had seven open deferrals (also called “write-ups”) when we got it, and by the time we landed after one leg, it had 11 open items. Most were pretty mundane: the FO map light was out, one of the hand mics didn’t work, and one of the autopilots was kicking off intermittently, which in turn led to a deferral of the autoland system. While we had the airplane, the printer decided to stop working, which was inconvenient, but by no means a show-stopper. That said, by the time we landed, we were happy to be done with it.

As often happens in a case like this, the maintenance folks use their computer systems to track not only what is wrong with a given airplane, but its flight schedule. They work with the network folks to get the airplane routed in such a way that it can be repaired efficiently, and without interrupting the schedule. With 11 open items, this one finished the day at one of our major hubs, where the mechanics were waiting on the jetbridge to take it to a hangar that had all the necessary parts to get it repaired and back to work.

We brought the mechanic up to speed on the issues we had found, and between us we made a few jokes about returning the airplane as a lemon. However, almost every airplane has something on it that needs to be addressed. It’s inevitable on a machine with a million or more parts, many of which move at high rates of speed or operate under extreme environmental conditions. In truth, the airplanes and their parts are incredibly reliable and robust. Unfortunately, not every station can stock every part, so the deferral process is what keeps the system running.

Most importantly, it keeps everyone safe, even if there is no real cure for the measles.