Expert: D. Norkus - 10/17/2006

Question
Hi

I was wondering why it is airlines have to turn the lights out when they land and take-off at night. A friend told me it is to conserve power, but I don't think that's very viable! Is there a legal reason why they do it, or a technical reason?

Secondly, and this is a silly question, but what would the acceleration times of a jet airliner be? (talking 0-60mph, and using maximum possible power, which I know would not be used in real life!) Would it vary from aircraft to aircraft?

Many thanks!  

Answer
Adam

I.
Light are absolutely NOT dimmed for "power conservation"! That would be like turning off the radio in your car so it would go faster- simply ridiculous. Aircraft electrical is run off engine driven generators, alternators or transformer rectifiers. Items using power from them have no effect on the thrust or power the engine produces. So you are right, it isn't a viable explaination.

During the take off and landing phases are when most emergencies happen. If the lights were on in the cabin, you would not be able to see the conditions outside the aircraft as well as if they were off if an evacuation were to become necessary. Evacuating into an unsafe condition like fire, water or an obstruction ect is not a wise idea! During the day the cabin lights can reflect off the windows making outside conditions difficult to see. At night, low lighting would improve your night vision making the dark outside much easier to see from in the dimly lit cabin. Hence, they are dimmed for both day time and night time takeoffs and landings to improve visibillity in case an emergency were to occur.

II.
The time for an aircraft to accelerate to a given speed would vary on a number of factors like: aircraft weight, temperature, density altitude and power setting used. You would have to know all those variables to truly figure out the time it would take.

You might visit the videos at www.flightlevel350.com. Search for "takeoff/cockpit view videos" and time a few different aircraft to see, as an example, of how fast they accelerate. "80 knots" (about 92 mph as 1 mi/hr=0.87kt/hr) is a usual call out, so that would be a good benchmark to use.

BTW-Max possible power is usually not used on most take offs to conserve engine life. A reduced power/thrust setting that is appropriate for that weight of that flight, the runway, altitude and temperature is used instead.

Hope this helps
Dottie