There’s quite a bit of chatter on social media about accident flight recorders.
One of the skills required by an aircraft accident investigator, and not often mentioned, is the ability to grapple with rules, regulations, and technical requirements. This is given that civil aviation is one of the most highly regulated industries in the world.
The story of the development of the accident flight recorder is a long one. No way can a few words here do justice to all the efforts that has been made over decades to ensure that this vital tool for accident and incident investigation does what it’s intended to do.
In fact, that’s the first technical requirement to mention for accident recorders. Namely, FAR and CS Subpart F, 25.1301: Each item of installed equipment must be of a kind and design appropriate to its intended function. That basic intended function being to preserve a record of aircraft operational data post-accident.
Aircraft accident recorders are unusual. They are mentioned in the airworthiness requirements, however they play no part in the day-to-day airworthiness of an aircraft. The reality is more nuanced than that, but an aircraft can fly safely without working flight recorders.
FAR and CS 25.1457 refers to Cockpit Voice Recorders (CVR)[1] and 25.1459 refers to Flight Data Recorders[2]. Both CVR and FDR receive electrical power from the aircraft electrical bus that provides the maximum reliability for operation of the recorder without jeopardising service to essential or emergency electrical loads. Both CVR and FDR should remain powered for as long as possible without jeopardising aircraft emergency operations.
Before drawing too many conclusions, it’s important to look at the above certification requirements in relation to their amendment state at the time of type certification of an aircraft.
If the aircraft of interest is the Boeing 737-800 then the FAA Type Certification date is 13 March 1998 and the EASA / JAA Type Certification date is 9 April 1998. Without wading through all the detailed condition, the certification basis for the above aircraft type was FAR Part 25 Amendment 25-77 and JAR 25 Change 13 [Note: EASA did not exist at the time].
FAR and CS 25.1457 and 25.1459 were in an earlier state than that which is written above. That said, the objective of powering the recorders in a reliable way was still applicable. There was no requirement for the CVR or FDR to be powered by a battery. What hasn’t changed is the requirement for a means to stop a recorder and prevent erasure, within 10 minutes after a crash impact. That’s assuming that aircraft electrical power was still provided.
So, when it’s reported that the South Korea Boeing 737 accident recorders[3] are missing the final 4 minutes of recoding, the cause is likely to be the loss of the aircraft electrical buses or termination by automatic means or the removal of power via circuit breakers. We will need to wait to hear what is found as the on-going accident investigation progresses.
[1] https://www.ecfr.gov/current/title-14/section-25.1457
John Vincent 