Plane Answers: Aircraft inspections, air filtration and stalls

Welcome to Gadling’s feature, Plane Answers, where our resident airline pilot, Kent Wien, answers your questions about everything from takeoff to touchdown and beyond. Have a question of your own? Ask away!

John asks:

Hi Kent,

I’m a aviation nut and I’ve always been curious about this question: What is the difference between a C and a D check? I know they both involve overhauling the aircraft.

My friend Ed Straigis has become my ‘go to’ guy when it comes to maintenance questions that come up here. Fortunately, he just happens to perform these overhauls you’re asking about.

Here’s his explanation:

A “C” check is a complete, heavy overhaul; the aircraft is opened up, all inspection panels, seats, galleys, lavs, overhead bins, side-walls and insulation are removed and the aircraft is checked for cracks and out of limit equipment such as hydraulic actuators and bearings.

The brakes and tires are replaced and the aircraft gets washed and polished and comes out looking like new. This usually takes about 3-4 weeks.

We don’t do “D” checks, since we’re allowed by the FAA to do repetitive “C” checks, with each “C” check being more involved than the one before. Our C checks usually occur about every 18-24 months depending on the number of cycles (flights) on the aircraft.

Other carriers that do “D” checks usually do them every 4-5 years. They require the aircraft to be out of service for at least a month or more. It’s about the same a a C check but more involved.

Since we do our “C” checks more often that other airlines and cargo outfits, we’re able to find small problems and fix them before they become major problems instead of waiting to discover the issue during a “D” check.

Thanks Ed. I have always wondered about that myself.

Mark asks:

Kent, I continue to really enjoy your writing. You may not know this, but I often wonder how the hours I am putting on the King Airs I fly compare to those of the typical airliner. My operation has 6 King Airs and we are flying them about 1200 hours per plane per year. How does this stack up against what a typical airliner does?

Thanks Mark,

I looked up the latest airplane I flew yesterday, a 767 that was built in June of 2003, to get a feel for the utilization of our airplanes. It has 22,500 hours right now and it has made 3555 flights so far. This represents an average of 4,219 hours a year or 11 1/2 hours per day.

It’s pretty easy to have efficient utilization when each flight averages over 6 hours.

Years ago, when I flew the 737, I watched the hours on those airplanes as they approached their first full year flying and found them to consistently be in the 3,500 hour range. That makes sense, because domestic aircraft are on the ground more often, loading and unloading passengers, which keeps the utilization down.

Hope that helps, Mark. I would have loved to have flown a King Air.

Joe asks:

Hello Kent,

I have a question about the air circulation systems. When I was younger and would fly pretty regularly across the states to see my grandparents, my parents always seemed overly cautious to see that my sister and I would get enough vitamin C and other remedies that day to keep us protected “from all the sick people on the plane.”

That mindset seems to have rubbed off on my older self. Whenever I fly, part of me is always scared that I’m going to catch the flu from that guy twenty rows up because the air on the plane is “recycled” and that his pathogens are going to inevitably make it through my nostrils–and worse, the crew’s.

Is this true? I know planes are pressurized, but how does the air filtering work? Are planes REALLY disease-spreading pastures of sorts, or is my being concerned unjustified?

Your parents were definitely on to something. My wife, a former flight attendant, would get sinus infections every few months when she was working. I haven’t had as much of a problem as a pilot, but our air is recirculated more often than it is in the cabin. This has less to do with keeping pilots healthy and more to do with keeping the avionics and instruments cool in the cockpit.

Boeing discusses these cabin air quality concerns, explaining that:

“Microorganisms do not pass through the filters of the highly efficient air recirculation systems on today’s jets. The system delivers a mix that is one-half outside air and one-half filtered recirculated air. It normally produces between 14 and 20 cubic feet of air per person each minute. As a result, the air supply in the cabin is essentially sterile and particle-free.”

Boeing does go on to admit that anytime you’re in close proximity to other people, there is always a risk of catching whatever your neighbor has.

In addition to the air filtration systems in all airliners, some even have “ozone scrubbers” that are necessary to fly across the Atlantic in the more northern latitudes. When our airline first started to fly to Manchester, England, from the east coast, some of the airplanes we flew didn’t have these ozone scrubbers, for the first few months, which meant we were restricted to altitudes of less than 35,000 feet until the entire fleet had been retrofitted.

Apparently Ozone (O3) builds up in aircraft cabins and can present a health problem–especially to crews who spend a great deal of their lives flying. According to this article, it’s the most significant issue affecting the air quality in airliners today.

Fortunately, passengers flying on the new 787 will enjoy a lower cabin altitude, more humidity and greater circulation of the passenger air, according to Boeing. I really hope I get the chance to fly it at some point.


Finally, Michele writes:

Hi Kent!

Another question from me, this one came up after hanging out at San Francisco’s Fleet Week air show a few weeks ago. What is stall speed? I had always assumed it had something to do with the engine, but I’ve been told it actually is more about lift and air speed. Can you clear up the meaning?

Good question Michelle, and one that is often messed up by the media.

A wing can only produce enough lift if it is flying fast enough. This airspeed will vary depending on the wing’s shape, whether the flaps are up or down, and how heavy the airplane is.

So when you hear about an airplane that ‘stalls’, it’s always related to a lack of sufficient airspeed.

Some people probably confuse it with the term “the engine stalled”, meaning failed to work. But that isn’t a term used in aviation. To be more clear, the media needs to understand that airplane engines don’t ‘stall,’ they simply quit or fail.

787 interior image at top provided by Ruthann O’Connor.

Do you have a question about something related to the pointy end of an airplane? Ask Kent and maybe he’ll use it for next Friday’s Plane Answers feature.