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

[unixronin.livejournal.com]

Assumptions have a bad habit of letting you down when you least expect it. Yeah, with a multi-engine aircraft, there should always be power available ... but that "should" can too easily go out the window about the time things start failing, like it did in this instance. Commercial airliners have multiple independent control systems, and the assumption is that even if both primary systems fail, there's still the mechanical backups — but my brother-in-law was working the tower at Sioux City, SD, when United Airlines 232 came in after the tail engine failed and took out all three redundant flight control systems, leaving the pilot with only the left and right throttles to fly the airplane with.



(bad typo. no donut.)