Airline Industry 106: Aircraft Maintenance by the letter

Class is now in session 🙂

Today’s Lesson: The Basics of Aircraft Maintenance

The aviation industry is heavily regulated. Most notably this is the case in the Commercial Aviation space – which is the category that most airlines fit into. One of the reasons it takes so long to develop a new type of aircraft is the rigorous certification process the design must go through before it’s approved to fly. Among MANY other things, the manufacturer must prove the aircraft can fly with only half of its engines functional. They must prove the interior of the aircraft will meet the flammability requirements (how the materials will be resistant to burning and if they do catch fire, how they burn). They must even launch chickens (or other suitably sized birds) into a running aircraft engine to certify how it will respond in the event it ingests a bird in flight – which is something that happens far more often than you might think! (Side note: The birds aren’t still alive when they do this – that would just be mean)

The same rigour that applies during the design and build of the aircraft carries over into the operation of the aircraft in commercial service. Aircraft are ‘worked’ hard and stringent maintenance requirements are in place to ensure the safety of the passenger. In an average airline, the average narrow body plane (your 737s and A320s) will fly an average of 11-12 hours every day. For context, that would be longer than a drive from Nairobi to Malindi or the same as a return journey from LA to San Francisco every single day. If you drove your car as much as a commercial aircraft is flown, it’s unlikely that it would last more than 3 years. Meanwhile, the design working life of an aircraft is approximately 20-25 years (give or take a few). So aircraft are meticulously tested in their design and build then they are rigorously maintained in their operations.

As part of the design process, the aircraft and engine manufacturers will develop a maintenance programme for each aircraft type. This is usually in the form of a Maintenance Planning Data (MPD) document which is approved by the aviation regulator in the State of Design of the aircraft (so the FAA in the US for Boeing, EASA in Europe for Airbus etc.). This MPD usually will not be aligned with the airline’s specific operational characteristics so, working with its own regulator, an airline will develop a customised scheduled maintenance programme that is based on the manufacturer’s MPD.

Maintenance Letter Checks

Most customised maintenance programmes will group scheduled maintenance with similar maintenance intervals into ‘blocks’. These blocks become maintenance event checks which are broadly split into two: 1) Line Maintenance and 2) Base Maintenance.

Line Maintenance is largely done at the airport (on ‘the flight line’) and includes daily walk around checks, pre & post flight inspections – done before and after every flight – and weekly checks.

Base Maintenance is carried out in the maintenance facilities hangar. It encompasses the heavier checks and tasks that will require more access to the aircraft’s inner mechanics. Base Maintenance checks are driven by the intervals of the tasks which comprise them and these are, in turn, driven by the utilisation of the aircraft. In the MPD, each task will have an interval based on either Flight Cycles (one take off and landing), Flight Hours (hours in the air) or Calendar (either days, weeks or months). Some tasks might have all three, in which case the most limiting is used. These intervals will be reviewed and revised by the State of Design regulator – mainly being increased as the aircraft type accumulates more hours and there is data to support claims of better reliability.

As a standard for consistency across the industry, the base checks are designated by letter, mainly A-Checks, C-Checks and D-checks. (B-Checks are no longer really used – their tasks were moved into A-Checks and C-Checks as the intervals were approved for increase)

A-Checks

These are the equivalent of taking your car for its regular service. The intervals will vary from 400-800 hours depending on the aircraft type. Based on the individual aircraft’s utilisation, this will occur every 4-8 weeks. During an A-Check they will carry out inspections of the interior and exterior of the aircraft, lubricate the moving parts, check oil and fluid levels – topping up as required and  replace filters and any components that fail functional tests.

A-Checks follow a cycle with increased tasks in each subsequent check until the cycle is reset. (usually after a C-check) i.e. a 2A check will include more tasks than a 1A and so on

C-Checks

C-Checks are carried out every 24-36 months – again, depending on the aircraft type and aircraft utilisation. Due to the calendar interval/limitation, even an aircraft that has not been flying will require a C-Check once the calendar time has elapsed before it can be operated commercially. The tasks at a C-Check include functional and operational systems checks, structural inspections, cleaning and servicing of parts and replacements of time expired or faulty components.

Like A-Checks, they follow a cycle of increased tasks that is ‘reset’ following a D-Check

D-Checks

These are also known as Heavy Maintenance Visits  (HMVs). These occur every 8-12 years. The aircraft is taken out of service for several weeks. It is often stripped of paint for structural inspections, large outer panels are removed to carry out detailed inspections of the airframe, wing and support structure. In addition, components undergo functional checks, repair, overhaul and/or replacement.

 

Larger components like Engines and Landing Gears have their own maintenance schedules and overhaul programmes. A deeper look will follow in future.

 

Before an airline can carry out any of the above maintenance on an aircraft type, they have to be approved by their regulator to do each of the checks. This includes an assessment of maintenance staff, tooling and facilities and procedures. Once approval is granted, it is audited and reviewed at regular intervals (usually annually) to ensure compliance. Approval can be withdrawn if the required standards are not met.

So, in summary, there are trained professionals inspecting and maintaining the planes you fly on, at approved facilities, more frequently than you take your car to the car wash 🙂

This concludes today’s lesson.