Cloudy: Twin engine airliners have a lower diversion rate than quads do.
You´re making a popular mistake, here.
A quad has a slightly higher chance of at least one engine failing just because of the higher number of engines.
The point you´ve missed, however, is that a quad will still remain
in a multiple-redundancy mode
, while the twin is in a clear and direct emergency
with no further safety margin. In addition, the twin will even have to operate its last remaining engine at a higher power level than the quad with its three remaining ones, which increases the probability of a failure of that last engine.
With an imagined engine failure rate of, say, 1%, the diverson rate
for the twin would be about 2% (rounded), for the quad it would be about 4% (assuming the quad would even need
to divert on a single-engine failure).
The all-engine-failure rates
, however, would be 0.01% for the twin, but only 0.000001% for the quad. Quite significant.
The first set of numbers tells you how probably you´ll be inconvenienced
, the latter is relevant for your survival
. When in doubt, I know which one I would
use for a decision.
Cloudy: Redundancy increases safety but so does simplicity. There is a trade-off.
That would only be true if the additional systems depended on each other
. Since practically all of them are working independently in parallel
, however, there is no or only a very small gain in reliability to be achieved. Nothing comparable to the reduced redundancy. It´s basically a political decision to accept
the increased risk.
Cloudy: 3. Airliners can glide a very long distance(can someone give us some figures?).
Not from an ETOPS 120 or 180 distance. Not by far.
Cloudy: Transport jets can land safely even if all engines were to fail at cruising altitude, provided a suitible landing surface is available. An Air Canada(I think) 767 ran out of fuel in the middle of nowhere and landed at an unused airport with no damage.
They were over Canada, which hardly compares to the open Atlantic or the Pacific in terms of "middle of nowhere". And it was only due to a very improbable set of circumstances
that they were able to land on an abandoned airstrip.
There was damage due to the inability to deploy the nose gear properly (in addition to a scraped engine pod, as far as I remember). If the plane´s tanks hadn´t been empty anyway (the reason for the emergency), there is the question if the higher weight would have made the landing a lot harder - and the fire that broke out after the landing a lot more dangerous.
The recent A330 gliding incident was also very
close to disaster. It was pure luck
they didn´t have to ditch because they happened to be close enough to the Azores.
Many people in here like to complain that the general public overestimates the risks involved in air travel. But naivity in the other
direction is much more dangerous. Having been incredibly lucky
for a few times doesn´t
mean that there is no risk
Cloudy: Within the US and Europe, there is little if any risk to life from engine failure at altitude - airfields and highways abound.
Just ask a few pilots whether they believe that bringing down a fully fueled airliner with all engines out on a traffic-heavy highway is their idea of an "acceptable risk".