Why are some mechanical problems found after boarding?

— -- Question: First of all, I want to thank the maintenance crews from the airlines for keeping these incredibly complex aircraft safely in the air. That said, as a 1K flier, I've noticed recently a rash of maintenance issues that are not found until passenger loading is complete. Some of these occurred on international flights where I expect the aircraft had some down time (perhaps even overnight) prior to being loaded. It may just be my perception, but why is that that many maintenance issues that potentially could have been addressed in between flights are not found until after the plane is loaded and passengers have to deal with an in-plane delay?

— submitted by reader crumbsteve

Answer: Maintenance crews certainly deserve praise. The fleet has never been more reliable.

Occasionally, a problem will be discovered shortly before departure. This is usually caused by the flight crew going through their pre-departure checks. During those checks, backup systems are activated to ensure they function properly. Pre-flight checks are executed while the passenger and cargo are being loaded. If a problem is discovered during the checks, maintenance is notified.

As for why the checks couldn't be performed during down time, there is a great amount of activity from the moment an airplane arrives to its next departure. Passengers and cargo are unloaded; the cabin is cleaned and refreshed (new blankets, magazines etc.); galleys are unloaded and restocked as catering is brought onboard. The process of loading outbound cargo (freight and baggage) begins, the lavatories are serviced, maintenance may perform a daily inspection, refueling takes place (this can take a significant amount of time depending on the uplift amount).

Pilots arrive about an hour prior to departure to begin their preparations (including a walk-around of the airplane), flight attendants arrive and perform cabin safety checks, cargo loading continues, passenger loading begins around 35 minutes prior to departure, and the flight plan information is loaded into the flight management computers and verified with air traffic control. At departure time all the cargo loading, fueling, and passenger loading is complete, the doors are closed and pushback begins.

It is a symphony of motion.

I am sorry to hear you have experienced a "rash" of delays. In general, airline performance has been improving, with recent data from the Bureau of Transportation Statistics showing 84.5% of flights arriving on time in the first four months of 2012, the best percentage since BTS has kept records. I hope your experiences improve.

Q: As a A&P (airframe and powerplant) mechanic, I would like to hear you overall opinion of aviation maintenance. Do you believe that maintenance should be moved overseas?

— Buckky

A: Overall, maintenance in aviation is excellent. The dispatch reliability for airplanes and helicopters is near an all-time high, proving the success of their maintenance programs. Additionally, there are very few maintenance-caused events in flight. Even with this excellent record, improvements can be made.