Aviation – When to Listen to the Computer

I’ve really been enjoying this series on aviation airliner accidents. When I used to fly in the 80s, I used to read accounts of various accidents in aviation magazines to see if there was something I could learn from them.

And I believe Lex’s account of his flying has helped some readers somewhere.

A few days ago, I wrote about another episode in this series that involved a simple tape covering the holes on the static port in the pitot-static system, and how that simple error compounded to the point it brought the airliner down.  That system, basically just a hollow tube sitting outside the fuselage or wing to collect the speed of the air, coupled with a few very small holes elsewhere to detect changes in pressure, is responsible for the primary instruments of the altimeter, air speed indicator, and vertical speed indicator. The latter tells the crew if they are climbing or descending and if so, by how many feet per minute.

And because in modern airliners so much of the computers are dependent on this simple system, if that simple system is blocked – by dirt, tape, ice, wasp nests, what have you, it really throws the computers off to the point that a pilot should ignore them, instead of trying to “correct” the attitude or speed based on the faulty readings.

Modern airliners also have a collision avoidance system, called TCAS, for Traffic Collision Avoidance System.

Modern aircraft also have a transponder, which also report their indicated altitude, position (via numbers that ATC tells you to “squawk” (a WW2 origin!) which is used by ground air traffic controllers. Of course if the pitot-static system isn’t working it reports an erroneous altitude.

TCAS utilizes this information among the aircraft and independently of ground control, will warn pilots of an imminent collision. It will tell one to descend and another to climb.

The problem is, what if the controller also sees this imminent collision and tells the pilot to do the opposite of what the TCAS is telling him?

Do you climb or descend?

This episode dealt with the accident between a DHL 757 and a Russian Tupolov 154 over Uberlingen, Germany.

As with most accidents there wasn’t just one factor. I liked an analogy Lex gave about slices of Swiss cheese.

Most mishaps involve many contributing elements – a chain of events, rather than a single decision. Sometimes you can line up five or six pieces of Swiss cheese and still see daylight on the other end – the holes line up.

As it was with Uberlingen. A lot of the fault rested with the overworked controller, who unknown to him had the functionality of his system reduced because of a maintenance crew working, and having to monitor 2 screens (and manage 2 sets of traffic patterns) because his colleague took a break.

But the main contributory cause was that on the Russian plane, his TCAS was telling him to climb, while on the corresponding DHL plane its TCAS was telling its pilot to descend.

And the controller was telling the Russian pilot to descend.

Let me clarify that.

In aviation radio communication, there are standardized phrases understood by pilots world wide. Talking on the radio in a plane has to be brief, succinct and with standardized phrases.

Not “hey Joe, go on out to Runway 32, hold it a while until we tell you to take off”.

It might be “Clearance to Runway 3-2 (that’s three-two – not thirty-two), and hold short“. To hold short means (at least in the 80s) to taxi up to the boundary of the edge of the runway and stop to await clearance to take off. There’s probably traffic on final about to land on that runway, and it is considered very bad form to be sitting out on that runway while someone else is trying to land where you are sitting.

Or, they may tell you to “Clearance to runway 3-2, taxi into position and hold“, meaning to get ready to begin the takeoff roll on the runway but do not go until told to do so. There’s probably still an aircraft ahead of you on that runway who hasn’t turned off onto a taxiway.

Actually, it looks like they are changing that phraseology to “line up and wait“.

Like I say, it’s been awhile 😉

Point is there are specific phrases both on the ground and aloft, and failure to understand them can mean the difference of life and death.

So when you are aloft and under the control of ATC and they tell you to “climb (or descend) immediatelyyou better damn well do it and don’t worry about acknowledging it back to them.

Just do it.

It’s that urgent.

Aviate, then navigate, finally communicate in that priority.

So, the harried Swiss controller told the Russian pilot to “descend immediately” while his TCAS, also sensing an immediate collision with the DHL 757, was telling him to climb.

What to do? You have only a few seconds – or less – to act. They were closing at over 800 mph.

So the DHL pilot was descending from his TCAS and the Russian pilot was descending from ATC instructions both to impact.

The program also brought up another similar incident in Japan involving a 747 and a DC-10 that could have been even more disastrous than Tenerife, where 583 people lost their lives in a collision on ground involving 2 747s.

In its report on the accident, published in July 2002, the Aircraft and Railway Accidents Investigation Commission called on the International Civil Aviation Organization (ICAO) to make it clear that TCAS advisories should always take precedence over ATC instructions. A similar recommendation was made three months later by Germany’s accident investigation body (the BFU) in light of the Überlingen mid-air collision. ICAO accepted these recommendations and amended its regulations in November 2003.

Standardization in aircraft operation protocols – in communication or operation, can and does mean the difference between life and death.


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