Nerds Only: About AC Power on an Airplane

Recently I mentioned Jay Rosen's complaint about Virgin America: the airline advertises normal 120 volt AC sockets at (most) seats, but Rosen said the power kept being cut off during his flight.

I wondered whether, as a technical matter, an airliner could actually produce enough power to keep a planeful of laptop users plugged in through a whole flight. Now I need wonder no more! Below and after the jump, two detailed replies by people in a position to know.

Short answer: No, you probably couldn't make this work. Short explanation: The necessary equipment is so heavy that, while it's fine for a train or bus, it's a real challenge for an airplane. For more, read ahead.

One reader says:

I ran some back-of the envelope calculations, and it turns out that providing guaranteed power to every seat in a large plane for the entire duration of a flight is quite a technical challenge.

Say you limited the power at each seat to 120V, 2A, a perfectly reasonable figure for charging a laptop - if everyone in a 500 seat plane drew 2 amps for the entire duration of an 8-hour flight, you would need the equivalent of ten or so Chevy Volt batteries, and an inverter the size of a large refrigerator (hopefully with some active power factor correction to prevent the "extreme sine waves" that Virgin references). Obviously, there would be less equipment for shorter flights/smaller planes, and everyone on the plane is not going to be drawing 2 amps for the entire duration of the flight, but if you want to guarantee power for everyone for the entire flight, it's what you need.

A simpler idea would be to have an alternator on the jet engine - but I have no idea if this is technically feasible, and it would have to be a fairly substantial alternator. Airline electronics typically run off of 400hz (rather than 50/60hz) because the transformers needed for 400hz power are much smaller and lighter than transformers needed for 50/60hz power (assuming the amount of power it can handle is equivalent), so you may need two separate alternators in this case.

Another idea is to have small inverters at each outlet, or row of seats, eliminating the need for a refrigerator sized inverter somewhere in the aircraft.

After the jump, a "translation" of the explanations from the Virgin American spokesman, to discuss the underlying technical realities. Now we know!

A reader with an aviation background provides this gloss on the Virgin America answers quoted yesterday. Normal type is what the VA person said; bold-face is the "translated" version.

Hey Jay - I discussed with one of our engineering managers and he informed me that the plugs have set thresholds where power will shut off to prevent surging. (The power inverters that provide power to the plugs are similar to the tiny plugs you put into the cigarette outlet in your car.  But in addition to being inexpensive, they must meet safety regulations, be lightweight, and oh, did I mention inexpensive?  That's why they have fairly low limits on how much power they can produce.)

Charging components within laptops have very sharp charging sine waves and the lower the stored power, the more extreme the wave.(So do the inverters that supply the power!  Switching Power supplies like the ones used for laptops depend on having the entire terrestrial power grid to absorb the spikes they generate.  The inverters used in car and aircraft AC outlets create their own spikes too.  When too much power consumption makes the spikes too high, the inverters will shut down to avoid damage to the components.  This is automatic and is part of the design.)

These thresholds are set for safety in accordance with FAA regulations. Similarly each aircraft is tested to proved full power at every plug before being certified. Our team is currently testing new designs to remedy this issue and we appreciate your feedback.  (Testing each and every outlet for full power sounds like we're doing something great, doesn't it.  When interference doesn't cause an inverter to shut down, yes, it provided full power.    The best real solution is to put inductors (big coils of copper wire wrapped around iron cores) in the circuit to absorb the power spikes and store energy. Inductors are heavy and expensive and we will never actually do that, but we will continue to consider it.)