Ask Captain Lim ~ All about Aviation

Hi Captain Lim,

I still love the site and enjoy your Weblog. I have asked questions before. I am a fearful flyer but with your site (and a little Xanex), I feel much better.

I was observing the preflight inspection of a Boeing 767 from the terminal and noticed that, at first, a mechanic would look at the plane. He would then check the landing gears under the plane, the engines, then at both the wings. I then saw the captain came out, put on a vest and did the same inspection.

What is the required checklist when doing this? What types of things are they looking for? Are both the mechanic and pilot supposed to do this, or just one of them? Or both do it - for two is better than one?

Also, when the planes are delayed due to "mechanical problems", are these just minor things like a light bulb, or could they have been fully functional parts like the engine or a flap.

Would they really repair major things right there on the spot, and just fly the plane after a major repair, like an engine problem, a fuel leak or a structural crack?

Thanks,

Stuart

Hi Stuart,

Yes, preflight inspection of a plane is part of the routine for all pilots embarking on any journey. No doubt the aircraft may have been certified suitable for flight and a complete inspection done by the engineer, it is normal for a flight crew member (either a First Officer and occasionally a Captain) to conduct a physical external check again before each originating flight.

There are many items in the Check List to be done as they conduct the "walk-around". This often-used term by aviators makes the task seem so casual but in fact, the pilots know what are the important things to look at. They check for impact damage, fuel, oil or hydraulic leaks, that static ports are not blocked (they caused a B757 to crash in Peru in 1966 because they were taped over by a careless mechanic) and many others. So, as a normal practice, external preflight inspections are performed at least twice before departure.

"Mechanical problems" can be anything technical that are either major or minor. Remember, an airplane can depart with various inoperative items - usually non-essential ones that have redundancies built-in, e.g. components in duplicate or triplicate. Whether it is a light bulb or something more critical, pilots are guided by two thick manuals known as the MEL (Minimum Equipment List) and CDL (Configuration Deviation List) to indicate whether they could proceed or not.

If, for instance, after starting up the engines, the captain had a generator failure. Could he depart? Is it a *go* or *no-go* item? He would then refer to the MEL. Any item listed is normally a *deferrable* component, meaning that the aircraft can depart with some restrictions. At other times, say, a hydraulic pump fails after start up, then it is a *no-go* item until the pump is changed. There are so many components in the planes for the pilots to remember and the manuals come in very handy.

Major defects are repaired at the hangar but minor ones can often be done on the spot at the aerobridge or apron.