[bruceboughton]

> I also know of instances where Boeing produced safer designs and had to lobby the FAA to get them to accept them. One example is using twin engines on overwater flights, rather than 3 or 4. The twin engines are statistically safer.

Interesting. Why do you think it is that twin engines are statistically safer? I thought redundancy was a normal part of the safety toolkit for planes.

[rplst8]

I think it's even true for single vs. twin engine planes. A lot of it has to do with - are the systems redundant, or are they both needed to properly fly the plane. Also whether the probability adds up in a serial or parallel fashion. I know a friend that pilots a single engine Cessna. Obviously one engine, but it also has no electronic ignition and four magnetos. https://en.wikipedia.org/wiki/Ignition_magneto The engine operates without the use of a battery because of this. I think the Cessna has a battery for auxiliary systems, but by removing the battery, spark module, and electronic timing/distributor from the critical path of engine operation, the chances of failure are lower. The four magnetos are for the four cylinders in the engine, but any one magneto can run the entire engine. In this case, the four items are redundant.

I think the first twin engine commercial airliner to be certified for over-water flights was the 777. It can take off, fly 6000 miles, and land safely with one engine and a full load.

[sokoloff]

Minor point: it's two magnetos, each magneto driving a separate ignition harness and spark plug in each cylinder.

Each system is tested in the pre-flight by grounding out (shutting down) the left magneto and making sure the right magneto can run the engine and all those plugs are firing, then repeating the test with the right magneto grounded (shut down) and making sure the left mag, harness, and plugs are capable of running the engine.

[Already__Taken]

I'm interested in how these redundancies have accumulated over the life of building aircraft. They're obviously for good reason. However the recent news pieces comparing electric aircraft, I wonder if there's a lot of efficiencies on the table for conventional aircraft if they could just drop in a modern engine like you might find in a race car.

[rplst8]

That's a good point. Gasoline engines have come a long way since the days of magneto operation, with variable timing, ignition, and direct injection (of which some piston driven aircraft may already use). Turboprops and turbofans (jets) are already very efficient compared to automotive piston engine aircraft. A 747 for instance gets around 91 mpg/person while buses average around 200 mpg/person - but, the drag forces on an aircraft are I think at least an order of magnitude greater.

[WalterBright]

I was told that more engines mean more possible failures, and that outweighed the advantage of redundancy. It's counterintuitive, but when you work the numbers for engines, it's true. It took Boeing a few years to convince the FAA.

[Already__Taken]

If you're further trying to wrap your head around it an example would be: https://en.wikipedia.org/wiki/United_Airlines_Flight_232

An engine failure manages to disable all hydraulic links and redundancies in the aircraft.