Writing the specification for an airborne recorder: the checklist

Start from the data, not from the box

The first section of a good specification describes the test or the mission, not the equipment: what the aircraft will do, what has to be measured, and what the recording is for — analysis after each flight, live monitoring from the ground, evidence after an incident. Every later line follows from it, and a supplier who understands it can propose something better than what was asked.

1. Inputs and channels

• The channel list, by family: analog with its sensor type and conditioning (strain gauge, thermocouple, IEPE, voltage), discrete, avionics buses (MIL-STD-1553, ARINC 429, ARINC 717), serial lines, PCM, Ethernet, video and audio.
• For each family, the count, the sample or update rate, the resolution and the bandwidth that matters: the figures that set the data rate.
• For buses: listen only, or also transmit? Both lines of a redundant bus? Every message, or a selection?
• For video: the number of cameras, their interface (HD-SDI, IP), the resolution and frame rate, and whether the picture is compressed on board.
• A growth margin: the channels the next campaign will add are cheaper as a free slot than as a second box.

2. Time

• The time reference: internal clock, IRIG-B input, GPS, PTP over the network — and what the recorder does when it is lost.
• The accuracy needed between channels, and against the other systems on board, stated as a figure.
• Whether the recorder must also give time to other equipment.

3. Capacity and recording

• The total data rate and the longest flight, with a reserve. The arithmetic takes five minutes and decides the medium.
• The recording format: an open one such as IRIG 106 Chapter 10, read by any compliant software, or a proprietary one — and then, with which software it is read.
• Start and stop: automatic at power-up, on a discrete, from a remote control; event marking if the crew must flag a moment.
• Behaviour on a power interruption: what must still be on the medium, and how quickly recording resumes.

4. Medium, download and security

• Removable cartridge or fixed memory, and how the data comes off: cartridge reader, Ethernet, on the aircraft or on the bench.
• Encryption of the medium if the data is sensitive, and how the keys are managed.
• Erasure: a quick erase between flights, a verified erase, physical destruction at end of life — three different requirements, often written as one.
• The ground software: which tool opens the data, on which computers, under which licence, and who must still be able to read it in ten years.

5. Crash protection, if the data must survive

• Whether the recording must survive an accident at all. If it must, name the standard: EUROCAE ED-112A for the full crash-protected envelope (or ED-112B where the programme requires it), ED-155 for lightweight installations.
• Which data must be in the protected memory, and how much of it: the last two hours, the last twenty-five, the whole flight.
• An underwater locator beacon if the aircraft flies over water.
• Recovery: who reads the protected memory after an event, how, and how fast.

6. Environment and EMC

• The standard and its edition — DO-160G, MIL-STD-810H, MIL-STD-461G — then, section by section, the category or the procedure.
• The real installation zone: temperature, vibration profile, altitude and decompression, humidity, fluids.
• Electromagnetic compatibility and lightning: emissions, susceptibility, induced lightning — the sections where a recorder installed near the radios is judged.
• The evidence expected: a qualification report on the same design, a statement of similarity with a qualified design, or new tests on this unit. Asking for new tests where similarity is enough costs months.

7. Power, installation and operation

• The supply voltage and the standard it follows (DO-160 section 16, MIL-STD-704), the power consumption, and the hold-up time on an interruption.
• Mass, envelope, mounting (tray or brackets), connectors and their keying, cooling (convection, conduction, forced air).
• The operator interface: remote control, status discretes, indicator lights — what the crew must see, and what it must not be able to do.
• Telemetry or live streaming, if the ground must follow the flight.

8. Documents and support

• The interface control document, the installation drawing, the user manual, the qualification and acceptance test reports, and the declaration of design and performance where certification needs it.
• Acceptance: what is tested on each delivered unit, where, and in front of whom.
• Support: how long, the repair turnaround, the spares, and what happens when a component becomes obsolete.

What to leave out

A catalogue figure pasted as a requirement: it rules out every other design for no reason. A technology instead of a need: "SSD" where the need is "removable, 500 GB, encrypted". And a document where every line is mandatory: mark each requirement mandatory or desirable, and ask for a compliance matrix with a comment on every line. The comments are where the answers differ.