The rules are stricter for single-engined helicopters, which should always follow the prescribed route.
BBC: London helicopter crash: What are the rules for pilots?
In 2011, Airbus stunned Boeing by announcing orders and commitments for about 1, 500 re-engined A320neos.
According to Bosch's calculations, a conventional internal-combustion-engined car can travel 1.5-2.5km on a kilowatt-hour (kWh) of energy.
This year, Boeing came roaring back, expecting to finish with firm orders for around 1, 000 re-engined 737 MAX aircraft.
The same design could be used in both Ferrari's front and mid-engined cars.
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The trend towards more twin-engined aircraft drives up the cost of engine development, by boosting the need for power and reliability.
The A380 is a four-engined, twin-decked, wide-body jet aircraft built at Airbus's factories throughout Europe and assembled at its facility in Toulouse, France.
The crew were flying their twin-engined aircraft on a night-time cross country navigation exercise out of RAF Kinloss in Moray on 13 April.
By analogy, imagine that you decided to build small engined cars because you predicted that gasoline was going to get ever more expensive.
However, multi-engined aircraft - like the one that crashed - can be provided with ATC clearance to fly elsewhere in the London Control Zone.
BBC: London helicopter crash: What are the rules for pilots?
Oh, and did you know that you can run a standard internal combustion engined car, with a very little tweaking, on that very hydrogen?
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Based on manufacturer estimates, the CrossBlue would attain 37 mpg highway and 33 mpg city, values that a gasoline-engined SUV would find hard to match.
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That is nearly four times more than the best petrol-engined vehicles.
Into that void steps the three-wheeled Torq Roadster from Epic EV, a team also responsible for that electric-engined DeLorean prototype we saw a couple of years ago.
These big-engined cars, particularly automatics, are proving hard to shift.
It is a rear-engined, four-door, four-seat car that will get around on 33hp--more pep than the Model T or the VW Beetle had when they drove onto the scene.
When Ferrari pulled the wraps off of its new 458 Spider convertible at the recent Frankfurt Auto Show it revealed another layer of deliciousness to the already scrumptious mid-rear-engined 458 Italia.
It really is extraordinarily easy to see the connection here: decades of higher gas prices have led to manufacturers producing more efficient engines and to consumers preferentially purchasing small engined cars.
Deliveries of the re-engined 737 are expected for 2017.
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SpaceX was quick to make the point that the ability to carry on despite an engine failure demonstrates a big advantage of multi-engined rockets like the Falcon (Space Shuttles and Saturn V moon rockets could both do something similar).
"The company, which was hip-deep in sorting out A380 development, simply hadn't foreseen the huge pent-up demand for a more efficient 250-seat airliner, and tried to take the easy way out by offering a re-engined version of its A330, " aviation journalist David Kaminski-Morrow, air transport editor of Flightglobal.com, told CNN in an earlier report.
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