Understanding Deferred Maintenance

Even experienced business aviation pilots and maintenance personnel may be occasionally stymied by the ins and outs of aircraft minimum equipment lists (MELs), which outline what equipment is necessary to perform a flight, and what equipment may be legally deferred under the regulations.

An aircraft’s configuration deviation list (CDL) specifies regulator-approved non-structural external parts that may be missing, yet the aircraft may still be airworthy.

For example, although many modern business aircraft typically are equipped with more than a dozen static discharge wicks, the CDL may specify that none of those wicks are required for a particular mission unless the flight will utilize the automatic direction finder or high-frequency communications.

“This was a ‘gotcha’ early in my career starting out as an avionics technician,” said Stewart D’Leon, NBAA’s director for technical operations. “With no static wicks in stock, [people] were saying, ‘Well, we can’t fly!’ Luckily, we had some great leadership that said, ‘Go look at the flight manual.’”

Flight crews may also understandably assume that inoperative ADS-B equipment could ground their aircraft, given that this system is required to operate in most U.S. airspace.

However, “there’s a caveat here,” said NBAA Maintenance Committee Chairman Greg Hamelink. “FAR 91.225 allows you to operate with ADS-B inoperative, and the FAA created a [website to] get an authorization for a deviation to operate without ADS-B,” he explained. “A pilot or flight crew member could do this deferral for maintenance.”

Operators also need to understand how Non-Essential Furnishing (NEF) lists work. These are lists of items that have no effect on the safe operation of the aircraft. Such lists may not encompass all that might break during normal operation, but they often provide guidance on how to handle a given situation without compromising a flight.

“The FAA provides for a process to defer items that are not on the list and come to the conclusion that the item is, in fact, a non-essential item, and thus a candidate for deferral,” explained Tom Atzert, president of Leading Edge Aviation Technical Services. “It’s essentially a flowchart in which you can determine, by answering a series of questions, if the issue falls under NEF.”

“It’s also important to remember the NEF is not a fix-all for everything that’s not in the MEL,” noted Mike Wuebbling, senior maintenance manager for Boeing’s Executive Flight Operations. “Some people tend to say, ‘Well, we’ll just create an NEF.’ It’s important that you follow the flowchart and check off those questions before doing so.”

That said, Wuebbling noted that the master minimum equipment list issued by the aircraft manufacturer often must be tailored to meet the needs of a specific flight operation or aircraft.

“If something doesn’t make sense, do not be afraid to pick up the telephone and call your center that’s responsible for your aircraft,” suggested Wuebbling. “Many times we’ve run into an interpretation that was in error. Once you identify that, you can send in a letter to the FAA and request that clarification to change that MEL.”