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"We will be delayed because of mechanical issues..."

Published December 31, 2023

The most annoying announcement at an airport gate, aside from “weather-related delays,” is most likely that there are “mechanical issues” delaying your flight. For one, it might mean you miss your connecting flight, have to rearrange your ride from the airport, or reach people to let them know you’ll be late to the party. On the other hand, if you’re a more anxious flyer, this announcement is probably on par with the in-flight announcement that turbulence is expected ahead. However, in both cases, you’re much safer than you imagine. We’ve talked about how planes can manage turbulence, even when it’s uncomfortable for fliers, just fine. Let’s look at some of the mechanical issues that might be occurring that can be delaying your take-off.
Many of the so-called mechanical issues have nothing to do with the plane’s ability to fly, and often concern the comfort, safety, and sanitation requirements of passengers and crew. One of those categories includes malfunctioning bathrooms, doors, latches, overhead compartments, galley implements such as coffee makers, heaters, and refrigerators, just to name a few. This can also include a broken seat belt on a sold-out plane that has to be replaced before take-off, or a piece of loose carpet that can be a tripping hazard and has to be taped down. Each plane type has a detailed manual that specifies which defects are considered issues that will halt a flight and need to be fixed before the plane can take off, and those that can be remediated temporarily or wait until the next scheduled maintenance. For example, if there’s a latch malfunctioning in a galley, the flight might still be operational, with that particular compartment not used on that flight or until it can be repaired. 
In that same category are issues with malfunctioning air condition units, window shades, entertainment units, reading lights, or seat recliners. They might range from annoying (no movies or internet on this flight!) to needing to be replaced for safety if they, for example, include exit row seats that are stuck in a reclined position or window shades that do not open. 
The next category of issues are those that affect avionics and flight control indicators, but do not usually mean that any actual flight controls are malfunctioning. Each display or lighted indicator is part of a redundant system, with double or triple instances of instruments and controls. If one of the instruments fails or provides erratic readings, maintenance teams will check to see if they can replace just that instrument, which can delay your departure. For example, if one of the landing gear indicators shows an issue, but the landing gear is fully operational, the malfunctioning light will be checked. The same is true for weather-related sensors and radar equipment. While the plane could technically fly with one functioning system, regulations require that redundancy is maintained. 
And finally, there are some actual plane-related or external mechanical failures that can delay your departure. While these are the least common, they do occur. Most often, pilots will have reported observations during flight, so when the plane is back on the ground, the maintenance crew can check for hydraulic issues, brakes, or landing lights, just to name a few. Because commercial planes undergo regular and rigorous maintenance check-ups and pre-flight tests, these issues are rare. In most cases of more serious concerns with engines, flight controls, or hydraulics, for example, planes will not continue to fly until they are repaired. It’s also possible that external doors, cargo hatches, or other seals are malfunctioning and need to be repaired. Another possibility are failures with auxiliary or ground power units, which are used to power the plane while on the ground and start a plane’s engines. If a ground power unit has to be exchanged, it can take a few minutes and the pilot might lose their spot in line and have to wait for another take-off position. 
So next time you hear that annoying announcement of mechanical issues, remember that most likely it’s a non-flight control issue. While it may not directly impact the aircraft's ability to fly, it can still cause those dreaded delays, as airlines prioritize passenger safety, comfort, and the overall operational integrity of the aircraft. But think of your comfort, too! Who would want to stare at a black screen during the entire flight or go without a nice hot cup of coffee before landing after a red-eye flight. And of course, safety always comes first. Even small malfunctioning pieces can become a safety hazard during flight.