While waiting getting off our “a little bit late” flight the cabin crew exhorted everyone to clean up after themselves in order to facilitate a quick turn around for the next flight. (Exiting the plane I saw that someone has been eating pistachios and dropping the shells on the floor. WTF?) Anyway, it got me to thinking - how many cycles of take offs and landings (or is it hours?) before some mandatory maintenance needs to be done before the aircraft would be unable to empty, load and go in the normal hour or whatever it is between flights? Also, does the pilot still do a physical walk-around before each flight? I don’t recall seeing that recently but then again, I’m not usually looking.
Pilot walkarounds are still required before every US flight. Except on Southwest which petitioned the FAA to waive that requirement to save time. So they don’t. Just another money-saving shortcut.
Checks are generally required based on a mix of cycles and hours. The least intrusive checks get done about every other day for typical use patterns. This would be things like engine oil & hydraulic levels, tire pressures, a careful once-over for dings and dents outside and loose trim or seating stuff inside. Oil & hydraulic levels are also checked by the pilots before and after each flight so they can get topped up without triggering a delay.
As a separate matter there’s a pretty in-depth check that happens before every trans-oceanic flight.
The airplanes are pretty reliable. We write up a maintenance discrepancy on the machine maybe once every 5-10 flights. Cabin issues are more common, maybe one per 2 flights for seat recline, loose trim, soiled seat (I wonder how that happened?) broken entertainment stuff, etc.
Once something gets written up it must be fixed (if major) or scheduled for fix (if minor)before the next departure. There’s a big book of malfunctions and required amelioration procedures and time-to-fix limits.
Somewhere in GQ annals I asked if the pilots actually kicked the tires on their walk round. I can’t remember the answers, but I still like and find reassuring the idea, the connection of man and machine that is understandable even to groundlings.
When I do it I don’t kick the tires; the pressure is so high there’s no point. But I do feel the air near the brakes to see how hot the last guy left them. Actually touching the brake assemblies might be a career-altering experience if the last guy really heated them up. They’re also filthy with brake dust, just like a car.
LSLGuy - did you ever find anything on a walk-around that was serious risk to flight safety? I would hope that such a thing would be caught before a final visual inspection. But I guess that’s why you do them.
I’m sure this tale has circulated through all the lil’ hangers on the civilian side of the airfield all around the world:
It was just a couple of dents on the left wing, what harm could come of it … the plane took off fine and flew well … maybe I was way over-reacting with the rental office, I’ll apologize when I get back down … so off to the matter at hand … practicing a basic stall … throttle back, nose up, slowing down … left wing stalls, right wing doesn’t … I guess it’s time to practice emergency spin recovery …
Lovely …
The FAA has requirements for aircraft inspections. They are called A, B, C, and D checks. A check is the most basic, a D check involves complete inspection of the fuselage and can take an airplane out of service for a few weeks. Airlines often use D checks to upgrade many items, the plane is going to be taken apart anyway. We get reports from these checks at my job at the big airplane company, especially if the check finds something wrong that happened during the assembly of the airplane. The checks are scheduled after a certain number of cycles. A cycle is one pressurization event for the plane. The number of cycles for each check also depend on where the plane is flown, those flown in salt air environments get checked more often. The Hawaiian Airlines 737 that lost it’s roof pushed this, that plane had it’s D check a year prior to failing. That check should have caught the corrosion problem that caused the upper skin panel to fail.
These FAA requirements: they’re required for whose/which planes? Every one that operates in US, i.e., if the airlines don’t/can’t show they’re meeting them, they’re banned from US facilities?
So if I fly a Boeing 727 on Air Good Luck out of Shithole, Bangeezee, Africa, serving only Lower Bangeezee, that 727 is not allowed in the US, but Air Good Luck still is, with its one Boeing 747 which meets FAA schedules?
(FWIW, I was looking at airfares NY-Tel Aviv, and found Air Kirghestan and Aeroflot at half the price of the next one up, so it got me thinking.)
Maintenance looks at the aircraft every couple of days. Except for Southwest, the pilot looks at it before every flight. So our inspection is often not so much the final one as the only one. That’s not quite true because in addition to our check the cargo/baggage loading crew is supposed to keep their eyes open for leaks & dents and such. The simple stuff they can easily recognize as not-right. More than once they’ve seen and reported something to me.
Over the years I’ve spotted underinflated tires or over-worn tires, low hydraulic or oil levels or accumulator pressures, bird strikes, surface dents from ground equipment collisions, nicks in engine blades from sucking up junk or rocks, hunks of plastic bag hung up on engine stators, minor engine oil leaks, minor hydraulic leaks, burned out light bulbs, improperly closed galley doors, and access panels with unlatched fasteners. And ice on the aircraft.
Almost all of those are trivial issues with not much way they can get serious. Most of these are problems of accretion where it’s not exactly broke; it’s just less than shiny-new ideal. So we fix (or record) the deterioration and press on.
There was one very badly underinflated tire that might have, if we were real unlucky, come unglued during takeoff. Which in turn might have snow-balled if the tire chunks went down an engine or holed a fuel tank. A general concern with tires is that when one fails, its mate(s) not only pick up a 33% or a 100% load increment but may have taken a couple shrapnel hits as well. That shouldn’t cascade unless there’s a lot more bad luck than usual. But “shouldn’t” and “doesn’t” aren’t always the same thing.
Ice on the airplane is a whole 'nuther kettle of fish. Overlooked that could easily be fatal. The good news is it’s pretty obvious when you need to be concerned and when you don’t.
how much of a “nick” can a fan blade have before it needs to be replaced?
Speaking very generally, a small nick/dent in the leading edge of a fan blade can be blended out with abrasives. Sometimes, the blade directly opposite the repaired blade will then be blended in roughly the same location, to maintain the balance of the fan assembly.
The engine manufacturer’s maintenance manual will spell out exactly how much material can be removed from the different “zones” of the blade, and what abrasive materials can be used.
To hang a number on your question, I’ve blended out damage as deep as 0.020" and 0.060" wide, on a 1st-stage compressor blade of a PT6T-3DF turboshaft engine.
ETA: Comprehensive ninja by JHBoom. Great info; thanks. Now that we have a non-lightplane A&P/AI on the boards I can let these ones ride.
That varies by engine model and the details of the nick size & shape. I don’t have access to the criteria. My job is to see it and call the guys with the big book of details. Those are the guys JHBoom supervises.
The usual “fix” is to take a plain old hand-file and file off any bent metal plus a bit more then create a smooth S-shape where the normal edge smoothly curves downwards towards where the divot was, then smoothly turns to parallel the blade face then smoothly curves back up towards the edge then smoothly curves back the other way to meet the undamaged edge.
The idea is to gently remove any bit of metal that may have been overstressed by the impact. Then leave a large radius curve so it doesn’t create any stress concentrations. Finally, do the work by hand so nothing gets heated enough to affect any heat treating. The area will get an eddy current inspection either before or after to detect any cracking.
This process is called “blending” or “blending out a dent”. Probably 10% of the underwing engines flying right now have a blended blade. It’s not a big deal. Once in a great while you see an engine with two blades with one blended dent apiece. But that’s real rare.
So you might end up with a filed-off area 5/16" deep at the deepest by 1-1/2" wide from end to end. Which represents the “repair” for what was originally a gouge maybe 1/4" wide by 1/8" deep. For scale, this is on a blade anywhere from 2-1/2 to 4 feet in span and 4-8 inches in chord. A typical modern engine has about 25 fan blades.
There’s also standards for how big a damaged area is too big and then a blade change is required. To directly answer your question, 5/16" x 1-1/2" is about the biggest blend I’ve ever seen, so I conclude that 1/4"x1/8" is also about the biggest damage before a blade change is required. I’ve also never seen two blends on one blade. So a second damage event on a blade probably triggers a blade change.
Blending can be done by a skilled worker in an hour with the engine on the aircraft. Changing a blade can be 2 or a 10 hour project depending on the engine type and how much needs to be disassembled to get at it. Sometimes a blade change triggers a fan re-balance. They tell me that’s a big job. So clearly there’s motivation to blend if they can, change if they must.
That’s interesting. I was watching a documentary about the De Havilland Comet - the world’s first passenger jet. It was extensively tested before being put into service but after several crashed for no apparent reason, they were all grounded. Further checks revealed that the failures were due to metal fatigue which happened when the pressurised cabin expanded and contracted. The designers had installed square windows and the failure points were at the corners. A round window version was produced, but the public confidence was gone so the whole project was abandoned.
I believe that many of the patents were subsequently sold to Boeing.