Radar Loss of Separation Alerts, ELTs and Smoke Alarms

How are these things related? Well the simple answer is that they could save your life. An other answer is that they are all complex technologies which aim to solve pernicious problems by alerting humans to danger. They also share a property with all complex technologies, bugs. Bugs in an alerting system lead to two types of false alarms:

• False negatives, where a real problem goes undetected, or unreported by the technology.
• False positives, where an alert is issued when there is no problem.

False negatives lead to law suits and so get prompt (in the broadest sense of the word) attention from regulators and manufacturers. False positives are often seen as an inconvenience that can be most effectively dealt with by the human when alerted. Unfortunately humans usually have a direct response when face with a high rate of false positive alerts.

The tech that cried wolf

When smoke detectors first became available they had a very high false alarm rate. Often residents would solve the problem by disabling the smoke detector with predictable and unfortunate loss of life. However legally mandated installation in residential buildings created a huge market. Competing suppliers drove improvements to the technology that has brought the false positive rate down to a very low level.

Aircraft ELTs (Emergency Locator Transmitter, which Search and Rescue can use to find missing aircraft) come in three flavours by TSO designator (the Technical Service Order or TSO is one of the ways equipment can get approval for use in an aircraft, and is required for ELTs); C-91, C-91A and C-126. Due to a whole bunch of history that I won't go into, the C-91 and C-91A units were (in my personal opinion) never properly engineered to do the job. This has lead to a staggering number of false positives, and an appalling though not widely reported number of false negatives. While the aviation community was wrestling with the limitations of C-91/C-91A units, the maritime community was adopting the Category I and II EPIRB which would later be imported into aviation as the C-126 or 406MHz ELT. Through some creative and dogged pursuit of their mandate the aviation SAR community has never given in to the temptation to ignore this particular boy crying wolf. But the system has. Citing the better performance of the 406MHz technology, and the high cost of maintaining the 121.5MHz COSPAS/SARSAT packages processing of the 121.5MHz signal will cease 1 February 2009. If you read enough material on the subject you will also encounter references to the high false alert problem.

So why not just buy a C-126 ELT? This is a very complex issue. If you are involved in general aviation this is something you should educate yourself on and decide for yourself what to do.

Lastly, in the aftermath of a mid-air collision over La Mesa California Feb 8, 2006 it appears that controllers have displayed the same human frailty as early smoke detector owners. I am glad to see that at least some attention was given to fixing the false alerts. If the response is only to require controllers to forward alerts to pilots as the technology issues them, it would only move the problem from controllers (who are all professionals, current and experienced in assessing the traffic threat, of whom we can expect more) to pilots (who may only fly recreationally, be inexperienced in dealing with high traffic volume and the technology quirks, and finally just as able to fall into the trap of ignoring the boy who cried wolf).

If we want safe homes and safe airspace we need to deal as proactively and diligently with false positive alerts as we would with accident investigation reports. Replace the cranky smoke detector and give it fresh batteries, check ELTs after landing or servicing the airplane, be vocal when safety technology promotes more annoyance than safety.