How pilots keep track of aircraft defects

Everyone hates a delay. Passengers are late for connecting flights, airlines get fined and pilots miss yet another family event. There are no winners -- except for maybe the retail outlets in the terminal. It's even more frustrating when the delay is due to a technical problem with the aircraft.

Airliners are machines and machines break. They are complicated structures comprising of hundreds of intricate systems. The more systems and components you have, the more there is to go wrong and fixing them can take hours or even days. Getting aircraft back in the air quickly, but most importantly, safely, is the priority for airlines.

As a result, because safety is everything, aircraft are designed to be able to fly safely with not just a single defective item, but with many of them.

So how do we keep track of faults in the aircraft and how do we know which ones are safe to fly with and which ones are not?

Aircraft maintenance log

How many of you reading this right now know full well that the next time you jump in your car a fault light will illuminate? Maybe it's either the tire pressure or the screen wash. Maybe it's an alert letting you know that it's time for a regular service? You know that the same light has been coming on for weeks.

Keeping track of faults with your car is pretty straightforward. For the most part, you drive the same car every day. You know that you need to inflate the tires. You know that you need to top up the screen wash.

However, what if you had a whole fleet of cars? What if those cars were being driven by different people each day? All of a sudden, it becomes a little more difficult to keep track of what work needs doing.

You hop in a car one morning that you haven't driven for months. You have no idea what's wrong with it. Maybe the brake pads need changing and all of a sudden it's a safety issue.

Airlines, pilots and engineers face the same problems on a much bigger scale, with far greater consequences if the faults are not managed properly.

Read more: How do pilots control an emergency evacuation?

To enable engineers and pilots to know what the technical status of an aircraft is, each airframe must legally maintain an aircraft maintenance log (AML). Traditionally, this is a large paper book that logs the details of every flight flown, every technical fault that occurs and the work undertaken by the engineers to rectify the problems.

At the end of each flight, the captain must fill in the details of the flight, including the fuel loaded and the flight times. This enables engineers to keep a track of how many hours and flight cycles the aircraft and the engines have flown. In addition, the captain must detail any defects with the aircraft which may have occurred during that sector. This includes faults in the cabin.

However, on the 787 Dreamliner which I fly, Boeing has done away with the paper logbook and has incorporated an electronic logbook (ELB).

Electronic logbook

Paper logbooks are cumbersome, they get damaged and, believe it or not, they get lost. Engineers are only able to find out what is wrong with the aircraft when it arrives on the stand and must remain on board the aircraft to complete entries before the aircraft can depart.

The ELB changes all that.

Using the aircraft communication systems, the ELB is able to feed real-time information about the technical status of the aircraft to various departments within the airline's operational infrastructure. This is advantageous for a number of reasons.

Firstly, it gives engineers more preparation time to rectify a fault when the aircraft lands. The correct spare parts and equipment may not always be readily available. On a long-haul flight, the ELB could give engineers many hours notice of a fault, enabling them to be ready with the correct parts on the aircraft's arrival. This has a huge effect on the on-time performance of the subsequent flight.

In addition, if the fault requires significant work, the early notification from the ELB allows the operations department to plan ahead and make schedule changes. It may result in the aircraft being pulled from service and using another one from the fleet. The earlier the facts are known, the sooner a decision can be made and the smaller the delay will be.

Logging defects

When a fault is detected on the aircraft, it is the duty of the captain to ensure that it is properly logged. When using a paper logbook, what is recorded by hand is the interpretation of the crew. The problem with this is that there is often no reference to the actual defect code, leaving engineers a tough job on deciphering what the problem actually is.

When using an ELB, the system automatically correlates entries and provides standard definitions for problems. This removes any ambiguity and makes it easier for engineers to understand what the issue is.

When entering a fault, we can either search for a word, for example, "brakes" or use the expanding menus to find the exact problem we have experienced. For example, if we feel that the brakes are pulling to one side whilst taxiing, we can find and log that exact fault as seen in the image below.

Read more: How pilots deal with fuel leaks

Traditionally, with a paper logbook, there would be a separate book for defects in the cabin such as seats which won't move or ovens that won't work. Whilst none of these are a problem from a safety point of view, they can have an impact on the customer experience. Unsurprisingly, the ELB brings benefits here, too.

The flight attendants are able to access the cabin part of the ELB without the need to go to the flight deck. Situated at the entrance doorway, the cabin attendant panel allows crew to enter any defects during the flight so that the arrival team can get an early warning of any problems.

For example, if the first-class cabin is booked to be full for the next flight but two of the seats are broken, there could potentially be two very unhappy customers downgraded. With an early warning of the problem, plans can be made to either get the parts ready to fix the seats or to give the customer other options with their travel.

Deferring defects

In an ideal world, all defects would be rectified before each flight. However, the reality of this is that almost every flight would be delayed to give the engineers time to do their work. From a commercial point of view, this is less than ideal.

As a result, a compromise must be found between the punctuality of the flying schedule and the safety of the aircraft.

When manufacturers design an aircraft, they know that dispatch reliability is a key part of the commercial success of a project. As a result, aircraft are designed in a way that there are back-ups for most systems. In some cases, there are back-ups for the back-ups.

Not only does this provide extra layers of safety whilst in the air, but it also enables aircraft to safely depart with some systems inoperative. This drastically reduces delays by allowing engineers to sign off a system as inoperative until it can be fixed at a later date.

All these details are recorded in the AML so that pilots are aware of what defects are affecting that particular aircraft.

However, how do we know which defects are safe to depart with and which ones are not? For this, we refer to the dispatch deviations guide (DDG).

Dispatch deviations guide

The DDG is a comprehensive list of every possible defect which could occur on the aircraft. Within the DDG there are two sections. The minimum equipment list (MEL) and the configuration defect list (CDL).

Simply put, the MEL details aircraft systems which are inoperative and the CDL details parts that are missing from the aircraft. For example, the MEL contains information for an inoperative brake system and the CDL has information for a missing external light.

When a defect is deferred by an engineer, it is done so under a particular reference code from either the MEL or the CDL. As part of the DDG entry, depending on the importance of the system or part, there is a time frame in which the defect must be fixed. This is indicated by a letter.

For category A items, there is no fixed time window in which the defect must be fixed. Category B items must be fixed within three days, category C items within 10 days and category D items within 120 days. Naturally, the more important the affected system, the sooner it must be fixed.

Read more: To land or to divert? How pilots decide the safest option

To access the ELB, engineers have a number of options. The most obvious is to sit in the pilot's seat and use the screens to access the ELB directly. This is fine when the aircraft is in the hangar, but when we are trying to set the aircraft up for the flight, it’s not particularly convenient to have an engineer in our seat.

To solve this problem, there is a data port next to the observer’s seat at the back of the flight deck which allows a laptop to be connected to the ELB system. From here engineers and pilots can continue their work without getting in each other's way.

However, even this second method requires the engineers to be onboard the aircraft to complete their work. With the need to remove the air bridge or steps, disconnect ground power, etc. before departure, precious minutes can be lost waiting for the engineers to leave the aircraft.

To save this time, the 787 has a wireless connection which enables engineers to access the ELB via a laptop whilst sat in a van on the ramp. This allows the aircraft to be fully prepared for departure whilst the engineers complete their work. As soon as they have finished, the aircraft can push back from the gate.

Pre-flight ELB check

When we first get on board the aircraft, one of the first things we do is check the ELB. Alongside any listed defect will be the DDG code. For example, an inoperative brake system is code 32-45-01.

We will then look up this code in the DDG to find out how this defect will affect the flight.

As an example, consider there are eight wheel brake systems installed (one per main wheel) but we only need six to depart. However, there are certain criteria which must be met to allow us to do so. These are either a maintenance procedure (M) carried out by the engineers and/or an operational procedure (O) performed by the pilots.

In order to depart safely and legally, these criteria must be fulfilled.

Bottom line

Safety is the number one priority for your pilots. It is our responsibility to get you safely to your destination. However, we are also aware of the commercial requirements to operate the aircraft in a cost-efficient manner. As a result, a compromise has to be made between getting defects fixed and getting you to your destination on time.

By making sure that we operate with strict adherence to the DDG, we can ensure that safety is never compromised, even if there are items on the aircraft not working properly. The electronic logbook on the 787 not only helps us keep track of the faults, but also helps to minimize delays by giving advance warning of technical issues to the staff on the ground.

At the end of the day, we want to get home to our families safely. If we do that, then so do you.