A modern jet airliner is a brilliantly designed assembly of a million or so parts flying in close formation. Given the vast array of systems and ancillary parts that make up the aircraft, it is remarkable indeed that the dispatch reliability of these machines approaches 98 percent. In other words, in 98 departures out of 100, the jet is capable of doing its mission. Every now and then, something breaks. That is when SWA Flight Crews get involved with SWA Maintenance to either get the issue fixed or deferred so that we can fly with this minor inconvenience to the next Maintenance (MX) station. The Federal Aviation Administration (FAA)-granted ability to fly with certain parts of the aircraft either not installed or inoperative allows us to keep at our primary mission: Getting our passengers safely and efficiently to their destinations.
This FAA grant comes in the form of the Minimum Equipment List (MEL). Every airliner has an MEL dedicated to it's specific aircraft type. The MEL grants allowances to fly with certain pieces of equipment inoperative and, depending on the equipment, may invoke additional limitations or time limits before repairs must be made.
For example, if your radar is out, you are cleared to launch as long as it will not be required for that flight. A close examination of the weather along the route answers that question. Another example is an engine anti-ice system. Not a particularly essential tool in mid summer. Of course, essential items like engines and wings are non deferrable.
Just because we have a book that says we can go with certain items inoperative, doesn't mean we make that decision on our own. Pilots call their SWA Dispatchers in Dallas and notify them of the problem and then the Dispatchers call a Maintenance Technician in MX Control for a three-way phone conference between parties. That MX Technician is dedicated to one specific model aircraft and has a computer file showing a MX history of the aircraft, as well as any current write-ups. With the Captain, Dispatcher, and MX Controller all working together, they can verify whether or not there is MEL relief for this issue and whether or not weather or scheduling would impact the plane operating in a degraded status.
An example: Let's say one of the two air conditioning packs is inoperative. The MEL allows the plane to depart with only one pack working but limits the flight to 25,000 feet or below. MX Control might feel this means we can go, but the Pilot and Dispatcher might know of the five-hour flight time and weather issues along the route of flight, or perhaps, at the destination. Using this all-encompassing approach, the Team realizes the aircraft flying at an inefficient lower altitude will not have the fuel to make the five-hour flight without either stopping en route for gas or leaving freight or passengers behind. Based on all the variables, they make the best, most informed decision possible. This may mean they swap aircraft with another flight which is only scheduled to fly short legs all day.
This whole deferral procedure can happen very quickly if the problem is minor and does not require any immediate MX attention prior to departure. In fact, I had one on my last flight.
At an out-station (no Company MX) during the cockpit pre-flight, we noticed the yaw damper was off. (Yaw dampers keep the tail of the plane from rocking back and forth at high altitudes.) Attempts to reset it were unsuccessful. Knowing this device was deferrable, I turned my cell phone on while writing up the problem in the aircraft logbook. By the time my phone was awake, I called my Dispatcher and got a patch to MX. In about two minutes I had a three-way agreement the problem was deferrable and not an issue for this leg (we would fly at a lower altitude where "Dutch Roll," the cause of the side-to-side tail wiggling, was not an issue).
After amending my release to show the current status of the aircraft had this inoperative yaw damper, I affixed a couple of yellow temporary warning stickers to the yaw damper switch, and we were ready to fly. The elapsed time was less than five minutes from the time of discovery.
At a SWA location where SWA MX was available, they would come and try to troubleshoot the problem. If the Technician was unsuccessful, the problem would be deferred by working directly with MX Control, and the Captain would advise the Dispatcher via phone of the issue and obtain an amended flight release (the main document showing the status of the aircraft and the most current routing for the flight with the very latest winds and fuel requirements).
Aircraft with MEL inoperative items are either fixed at the next MX base if it will not impact the next departure significantly, or for more lengthy repairs, they wait until the aircraft goes through an overnight at a MX base, usually within a day or so. If the MEL requires immediate repair (within a defined period), the aircraft will be rerouted to a MX facility where it can be repaired. This is often why passengers find themselves changing planes unexpectedly.
So, every part of that brilliantly designed aircraft you are sitting on need not be working. The redundancy built into modern jetliners allows certain systems to be inoperative with certain conservative stipulations. How often do I see a plane here at SWA with an MEL in the logbook? Rarely, if ever. Perhaps one a month or one every other month. That is a testimonial to the reliability of the machines we get from Boeing, as well as the dedication of the SWA MX Technicians who keep our planes flying in tip-top shape. It's no wonder they are at the very top of their field of aviation maintenance professionals.
No worries! Have a great trip!