777 engine problems; or, why the 3° approach is unsafe

[unixronin]

The BBC reports on a Boeing 777 that crash-landed at Heathrow Airport on January 17.  Crash investigators have found that the 777's engines were still running, but failed to respond when the pilot tried to throttle up two miles out and at 600 feet altitude.  The aircraft was not low on fuel.  Six previous engine failures have been reported in 777s since the type debuted in 1995; not a high rate, but high enough that they're beginning an intensive analysis of the entire fuel system.

Here's the handy diagram from the article (vertical scale is exaggerated in the diagram):

So, why does this mean the 3° approach is unsafe?  For that matter, why is a 3° approach the "standard" airline approach?

Well, basically, it comes down to this:  Airlines fly a 3° approach because a steeper descent may make the passengers feel uneasy.  But there's a problem:  the 3° approach is below the glide path.  It takes power all the way in to maintain the correct descent rate.  Lose power on final — like this 777 did — and you have a serious, often fatal, problem.

Military pilots fly a 9° approach.  Nine degrees vs. three isn't really that much steeper.  But there's a crucial difference:  On a 9° approach, you're on the dead-stick glide path.  That means you're not using power to control your sink rate, you're gliding with the throttles at idle.  You can lose an engine, or all engines, on final and scarcely care, because you can still glide all the way to landing with no power.

But a 9° approach "may upset the passengers", or so say the airlines.

I don't know about you, but if I were an uninformed airline passenger and was given the choice of a safer, steeper approach, or a shallower approach that might be a little more comfortable but risks crashing if the aircraft loses power on final approach, I know which I'd pick.

A hearty "Job well done" goes to British Airways Senior First Officer John Coward, by the way. Despite losing engine power less than a minute before landing, he put the bird down without a single serious casualty among the 136 passengers and 16 crew.  (One passenger broke a leg while evacuating the aircraft after the crash-landing.)  They say any landing you walk away from is a good one, and 151 people out of 152 walked away from that one.

Comments

[metahacker.livejournal.com]

Hmm...let's see. I've been flying for 30+ years on airplanes. Steep descents scare me. Never been in a crash. Never been in an incident. I think the low probability event is overemphasized. Military jets are trained to land when dinged up, low on fuel, under fire (9 degree is much faster!), etc. Completely different set of priorities.

I do wish they'd talk more about how the pilot managed to keep that tube upright on no power as it plowed across a field. Freaking amazing, is what it is.

And also perhaps talk about why the plane "suddenly had no power". My personal guess is "software", but perhaps that's a sausage-factory-worker problem.

[unixronin.livejournal.com]

I'm thinking software problem myself, considering the 777 is a fly-by-wire bird.