The face of mobile computing is changing. Freedom from having to carry multiple battery packs while flying with a laptop, in particular, is close to becoming a reality. In the next few years, a handful of airlines, most notably Delta and American, will be outfitting selected aircraft with power ports in first class and business class seats.
However, as with any new technology, a few kinks have to be worked out. Surprisingly, in such a highly regulated industry like commercial aviation, the specs for in-seat power have yet to be defined, even after two years of beta testing. Furthermore, it appears that the specs are actually happening after the fact, and that they are being drafted using the installed base of product as "justification" of how things should be in the future. This has the flavor of some "reverse-engineering" of the spec. Sound engineering, and often common sense, are being put aside in favor of keeping one or two airlines happy.
While select airlines are constantly rolling out new planes with an In-Seat Power Supply System (ISPSS) that was the only product offering in the aviation marketplace a year ago, the Airlines Electronic Engineering Committee (AEEC) is just now setting an output voltage standard for that power port. Other power system vendors have stepped forward and proposed changing the existing 15-V dc system to a range of 11 to 16 V, or even moving the aircraft-standard voltage out of the SAE car-voltage range.
Surprisingly, even though the in-flight power system was supposedly designed with the road warrior in mind, and is advertised as laptop-traveler friendly, the specification issues don't appear to reference how laptops work, and at what voltages. Instead, the AEEC's Cabin Equipment Interfaces (CEI) Subcommittee is approaching the ARINC Specification 628, Part 2, as an exercise as to whether or not an aircraft power system at the seat should look like or mimic a cigarette lighter in an automobile .
A Typical Installation
A typical aircraft ISPSS installation (like those identified as Proposals "B" and "C" in Table 1) is relatively straightforward. Illustrated is an overview of the system components (Fig. 1). The engine-driven generator provides electrical power to the entire aircraft. A segment of the airplane's total electrically generated power is allocated to the ISPSS. The Federal Aviation Administration (FAA) mandates that 100 W per passenger seat is the allowable maximum. Some older aircraft types were not designed for today's power-hungry devices in the passenger cabin, so there are airframes that have load schedules that may deliver less than 100 W to each ISPSS as input.
The 400-Hz, 115-V ac power from the generator's bus is controlled by a load-limit module that restricts the allowable current the ISPSS has available. This current-restrictor governs the total power allocation to all passenger seats. One reason why airlines are installing power ports only in first and business class seats is because there simply isn't enough power on the bus to support more than 100 to 120 seats with laptop power ports simultaneously.
A system controller, accessible by the cabin or flight crews, is used to manage power distribution to each seat. In every seat, there is an ac-dc converter that outputs 15 V dc to the power ports. The armrest usually has two electrical receptacles, one for each passenger.
These under-the-seat power supplies are constantly in standby mode. When a passenger plugs in a laptop, two of the four power pins in the AEEC-approved Hypertronics D-series connector short to indicate it is in-use, and the power supply wakes up (Fig. 2). A small LED indicates that power is available.
The passenger accesses the aircraft power system with a dc-dc adapter. This adapter converts the 15-V dc output of the seat's power port to the laptop's input voltage. Some airlines are opting for a loaner program, and are providing passengers with a limited selection of power adapters. Most airlines require passengers to provide the correct dc-dc converter.
Power management is rather rudimentary. The system controller monitors the seat-by-seat utilization of the power ports. As more passengers "plug in," the manufacturer's pre-set limit of 90% utilization is reached, and all seat ports not in use are disabled. It is difficult to calculate how many laptops any aircraft will actually accommodate. Even though the airplane features 120 power ports, a smaller group of high-powered (60 W, or more) ports could max out the system before all the plugs are used. Since the system may lock out any laptops on standby or sleep mode, airlines may find themselves dealing with unhappy passengers who are competing with one another for the power system's attention.
Now There Are Three
While the airlines have not had much choice of vendors who supply ISPSS technologies, two companies have thrown their hat into the ring and are submitting proposals to the AEEC/CEI Subcommittee. This subcommittee is charged with writing the specifications and standards for anything electrical or electronic in the aircraft cabin.
Table 1 also depicts the most important characteristics of the three proposals that the AEEC/CEI Subcommittee will vote on this month. In the interest of clarity, the table has been confined to the features that most directly impact the laptop.
Systems B and C are so similar that they will be considered here as one in the same. The only differentiator that will impact the specification is a subtle shift in the power port's output voltage. Proposal B wants to broaden the existing tightly defined 15-V dc output to 11 to 16 V dc, which will not dramatically impact laptop design or performance.