Electronic Design

Standards And Specs For In-Seat Power On Aircraft Still In The Clouds

You May Now Have A Chance To Assist In Deciding How Airlines Will Supply In-Flight Power To Laptops And Other Passenger Electronics.

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.

What it will impact is the overall system performance and power utilization of the aircraft's power grid. The present power delivery system is load-limited at the main bus to 100 W (or less) per seat. As suggested, over time, this may impact the number of users who can access power. By lowering the voltage, from 15 to 11 V, there will be a decrease in system performance. This will be created by an increase in dc-dc conversion inefficiencies, as the intermediate dc-dc adapters boost to the higher voltages (18 to 20 V dc) required by many notebook computers. Power losses by boosting to 19 V dc, from a base reference of 11 V, rather than from the present 15 V, will exact a toll.

Admittedly, a few dc-dc adapters converting from 11 to 19 V, at 80% efficiency, won't impact cabin systems' power utilization. But, with more than 100 such adapters running inefficiently for 4 to 5 hours at a stretch, the aircraft's generator has to work harder just to make up for the lost power. With an estimated 178,320,000 hours of in-flight laptop operation each year, the potential energy impact of an inefficient converter design can be considerable.

The adapters are fall into two classes here. The automobile dc-dc adapter is a generic, inexpensive unit that one may find at an auto-parts store. These low-cost units typically operate at 70% efficiency at loads below 1 A. Above 1 A, the power losses are even more dramatic, so 70% is more a best expectation, instead of the 50% or worse that these devices would experience in the real world at a laptop's 2- to 4-A rating.

The other class of dc-dc adapters are supposedly designed for aviation use. These are more power-friendly, as a group, but not all of the five manufacturers of these devices are created equal. The efficiency profiles of the three systems under consideration are shown (Table 2). We have seen "airline-standard" adapters with a total power limited to 20 W, so the corresponding chart may be optimistic for specific models and brands.

The chart represents a typical power system, and the baseline for available wattage is the FAA-mandated, 100-W maximum per seat. Systems B and C suffer from not only very low inherent conversion efficiencies (75 to 80%), but also the downstream losses created by connector losses of 2 to 5% (more on this later). The connector loss reduces that 100 W to 71 W, then the intermediate dc-dc adapter relinquishes another 15 to 30% of the available power to each seat.

System A has a definite edge in the power-efficiency arena. By avoiding voltage losses across inadequate car connectors, and by not using secondary adapters, the maximum 93 W from the power bus is passed on to the laptop. How does this happen?

All commercial aircraft have two "standardized" voltages throughout the cabin. There is a 110-V ac service available, as well as 28 V dc for lighting and other low-voltage cabin applications. So why wasn't the in-seat power established at the same 28-V dc standard? It seems absurd to pay the huge efficiency penalty of boosting from today's 15 V dc at the passenger seat. An Apple PowerBook at 24 V dc, for example, could almost operate at 28 V dc without any external adapter, since its supply has a voltage tolerance of 2 volts

Since it is always converting downward in voltage, System A uses the aircraft's 28-V system to optimize its power efficiency. The benefit is that it automatically configures itself to match the required input voltage of the "Personal Electronic Device" (PED, the term used in the aviation industry for any passenger-provided electrical or electronic equipment). The range of its output voltages also is important. Because the system can deliver power at levels in the 3- to 10-V range, this technology can fine-tune itself to power a PDA/HCE, in increments of .375 V.

Let's turn to other system features that are depicted in Table 1. PFC (Power Factor Correction) is quite good with Systems A and B, but System C is only a moderate performer with a PFC of 0.8. A more significant differentiator that notebook system engineers and designers should be aware of is the power management capabilities of the three contenders. System A uses an APM-style power management technology, so that the power at each seat on the plane is optimized at all times. Power-hungry PEDs and low-current-drain passenger equipment are distinguished by system A. This solves the dilemma of having to restrict the number of seats that can have active power ports at any time, as well as the total number of power ports on the aircraft. This allows those who travel coach class on aircraft equipped with System A to have the same opportunity to access the power system as those in first and business classes.

Systems B and C don't really have any power management, since both technologies employ a strictly "GO/NO-GO" approach. As seen in System C, when the load schedule exceeds 90% of the pre-determined capacity of the generator's bus, power is simply withdrawn from any inactive seats. To avoid this "use-it-or-lose-it" scenario, some savvy passengers will probably learn to set their notebooks in a continuous-loop diagnostics mode, and let the laptop sit there and do endless HDD read-writes when not in use. Even manually disabling any of the laptop's APM features during flight will guarantee that the notebook doesn't go into a sleep or standby mode, which the ISPSS could incorrectly identify as an inactive port.

One can imagine the consequences of having a laptop booted and with multiple apps open, only to have the power plug pulled. In this situation, the notebook's internal battery may or may not save the day. We'll also discuss some reasons why there may not even be a battery in the laptop while it's on a commercial airliner. The power management of System C goes even deeper into areas of notebook-user risk. Quoting from the specifications of System C: "Should the passenger connect to the system and exceed the system capability or cause a fault condition, the output of the socket will be automatically disabled." The description of this power management approach is best expressed as "ON/OFF only" in our comparison chart (Table 1, again).

Food For Engineering Thought
There are several situations that can trip the "kill switch" in Systems B and C. One is to have the laptop up and running, then insert a power-hungry PC Card, or an external peripheral, such as a CD-ROM drive. Power system engineers and laptop designers should pay careful attention to "worst-case" scenarios for total-system loads.

According to the specifications, any combination of hardware devices creating power demands that extend into the 50- to 55-W range can potentially trigger a shut-down in the ISPSS under the seat (Table 2). In Table 1, we have a line identified as "black-out" protection, which indicates a situation which few mobile computer system engineers have thought about— having the external power source go unexpectedly offline. Don't assume that the laptop's battery will be there as a life preserver. The passenger may very likely be boarding a plane after a long, battery-draining session of computer work in the waiting area of the airport.

Based on the picture of limited power in Table 2, with nonexistent power management and no fault-tolerant capabilities to protect the notebook (especially its data and OS), here are some suggested approaches to "airline-proof" your notebooks:

  1. Consider setting your battery's drop-out threshold voltage high enough to allow a "reserve" for emergencies, such as the ISPSS unexpectedly turning off, either in its fault-tolerant mode, or simply by not recognizing the laptop's power management when the equipment is in a sleep mode (which could be mistaken for an inactive seat port). Also, the audible prompt associated with low-battery states is almost impossible to hear over the high level of ambient noise on an airplane, unless you crank up the volume on the speakers for such alarms.
  2. Ironically, your software and/or hardware power management technologies may be inappropriate for airline use where Systems B or C are in place. As noted, if your laptop does too good a job of conserving power, the associated risk is that the ISPSS designs of Systems B and C could be fooled into thinking that the local seat power supply unit is in its "ON-but-not-in-use" mode. Think about having a user-selectable "Airline Travel" mode for power management. This might include leaving high-current-drain devices like the screen's backlight on. Perhaps there should also be a utility which keeps the hard drive or floppy motors duty cycling.
  3. If you do your own modified version of an OS, give serious consideration to a solid data-recovery mode. Windows NT, for example, usually puts you back to the same place after a power failure, while Windows 95 doesn't always allow such an elegant recovery.

If System A becomes the official standard for the airline industry, we can all breathe a little easier. Its power management actually enhances the energy-saving features in today's laptops, and it has fault-tolerant modes that eliminate any risk of a "black-out" at any time during the flight.

Adapters and Connectors
As seen in Table 2, intermediate dc-dc adapters have a definite negative impact on the total available wattage to the laptop. Of the five adapter vendors coming into the marketplace, their average power efficiency falls in the 80 to 85% range (that's being generous). So, if a typical adapter is used, the available wattage to the laptop's power-in port is a miserly 50 to 56 W. Connect one of today's "bells and whistles" multimedia machines to a power-limited adapter, and there's going to be an unhappy passenger. The dc-dc adapters that have overload protection usually rely only on resettable (or replaceable) fuses.

While Intel has proposed initiatives to the laptop industry to voluntarily limit the power consumption of notebooks to 25 W by 1999 (and that battery packs provide 44 W of power), the real-world trend seems to be going in the opposite direction. In 1994, notebooks averaged about 12 to 20 W, and that doubled in the last three years. Driven by "fat" software code, and consumer demand for desktop performance (especially in video and audio), it is anticipated that the average power consumption of notebooks in the 1998-1999 time frame will consistently be in the 50- to 70-W range. There are already laptops in today's marketplace that operate at well above 70 W.

In light of this, it appears that these secondary adapters are antithetical to continued laptop development because they severely restrict the available power to the equipment, while their inefficiency generates noticable heat.

On the Hot Seat
With all the problems associated with upconversion, you might ask: "What if the adapter were an inverter, and the output to the PED was 110 V ac?" What a neat idea it seemed when Toshiba and one or two other laptop manufacturers did away with the ac-dc "brick" by embedding it right in the notebook. In the office, at home, or in the car, there are no real safety concerns with a mobile computer that requires only an ac tape recorder cord as a power interface. But, at 30,000 feet, who wants an inverter's hot ac cord in one's lap, just as the person in the next seat spills a drink on you?

Another uncomfortable point to consider is that the use of nonmanufacturer-authorized power devices could harm a laptop's internal circuitry, and possibly cause data corruption or loss. Equally troubling for the passenger, it may void the product's warranty.

Table 1 indicates two connector variants. The Hypertronics connector is a DIN-style connector that is specific to airline power ports (Fig. 2, again). It's well-engineered mechanically and electrically, and has been used in medical applications for 14 years. The AEEC has endorsed and approved it as the official in-seat power port interface.

Unfortunately, marketing-department thinking has influenced the AEEC, when a standard car adapter was proposed last year as an acceptable alternative to the Hypertronics connector. Why? Because the aviation-based companies who were out selling in-seat power were promoting a system that offered passengers the convenience of a power port connector that was interchangeable with a passenger car's.

Connector companies, as well as car and aircraft adapter manufacturers, have not cast any thumbs-up votes for the automotive-style connector. Mechanically, it's an inelegant, unsophisticated affair. Despite their shortcomings, the AEEC Subcommittee was informed that, even though it had (on two occasions) voted against the car adapter interface, the car connector would still be part of the specification.

Not until the "electronic age" did the simple car cigarette-lighter receptacle have any "high-tech" application. As a matter of fact, the SAE Spec J563 still officially describes the receptacle as a "cigar lighter," which tells its age and original application.

SAE J563, in part, inadvertently fosters some of the mechanical instability of the plugs that fit into the specified dashboard receptacle. The SAE's dimensional specification for the "Cigar Lighter" is shown (Table 3).

The specification stipulates that any male plug manufactured as the "A" variant of the 12-V receptacle (20.93-21.01 mm) is not going to fit snugly in the "B" variant of the 12-V female receptacle (21.41-21.51 mm). At least one manufacturer of car adapters makes a small sleeve that decreases the I.D. of the "B"-size receptacles. Without this sleeve, this company's "A"-size male plugs tend to go askew and short out in a "B"-size receptacle. The "A"-size receptacles have been popular in U.S.-manufactured cars, while European cars typically have the larger-orifice "B"-size receptacles.

U.S. passenger vehicles are now coming on the market with two receptacles. There's the smaller "A"-sized, 12-V receptacle, with the classic heating element for smokers, and the second outlet is the larger "Euro-style" "B" variant. Electrically, these car plugs are prone to shorts and contact failures, and they often have significant voltage drops across the connector when there is not a very snug mechanical fit, as has been indicated by the 2 to 5% connector-related power loss (Table 2, again).

The SAE standard allows a maximum current of 8.0 A, which is well above the 2.5 to 4 A typical of notebook computers. Unfortunately, some "gray market" adapters that have voltages in the laptop range are designed for light-duty 500- to 700-mA range applications, such as cellular phones and other car accessories (personal CD-players, etc.). Put one of these low-power adapters between a 15-V-dc, 5-A in-seat power port and a high-powered laptop drawing 50 to 75 W and you can expect sparks to fly. In a $16 adapter you'll be lucky to find even a fuse for protection. These are some of the reasons why the AEEC voted down the car connector—twice.

Proposal "A" in Table 1 shows some promise for resolving many issues, including the connector. The car connector dilemma disappears with this system. Everything the passenger needs to interface with the in-seat power system is already on the plane. There is a Hypertronics connector available as an option, and that is only to allow airlines who have existing hardware at the seat to still use those power ports, but they are strictly alternate power outlets. Only a straight-through power cord is required when a Hypertronics receptacle is used.

About The Pending Vote
Anyone can go to an auto-parts or RV equipment store and purchase an inexpensive, noncertified, dc-dc (or dc-ac) adapter. What budget-minded road warrior wouldn't be tempted by a $16 gray-market car adapter, instead of the $89 to $129 "aircraft" adapters? The AEEC, the airlines, and the cabin crew are all powerless to prevent just that. The present specs may even invite it.

What has driven this 15-V standard, and the newly proposed 11- to 16-V spec, is a misdirected marketing opportunity. The message to the passengers from the airlines (and some factions of the AEEC rules-making committee), is that the power port should be the same interface as that of a car. What any engineer should see as too much of a temptation for laptop users to court potential electrical disaster, is seen by airline marketing as "passenger convenience."

In-Flight Safety For Batteries
While Table 1 lists a variety of system features, the AEEC vote will only focus on the first two items—system output voltage and the connector. But one of the three competing companies (proposal "A") is reflecting the concerns of the mobile computing industry by presenting a proposal to eliminate battery charging on aircraft.

To add to laptop maker's concerns, the Department of Transportation (DOT) is looking into battery safety on aircraft. Various sources have indicated that recent DOT meetings are focused on battery safety not as a matter of whether to charge or not, but whether or not potentially toxic battery materials should be allowed on aircraft at all.

Passenger exposure to battery venting, or heavy-metals contamination of the cabin interior may place severe restrictions on our laptop customers' rights to bring batteries onto an airplane. If the mere presence of batteries at 30,000 feet becomes illegal, the whole issue of charging becomes a moot point.

If the DOT's final resolution is one of "better safe than sorry," and batteries are banned on aircraft, the need for an optimized and sophisticated on-board power source becomes even more critical to the mobile computing passenger. If laptops can only fly without a battery pack, then the integrity and advanced features of a well-thought-out ISPSS will be the only safety net available to laptop users.

As an engineer in the mobile computer industry, it might be wise to not commit to any design or development which may be impacted by the in-flight power programs. Right now, everything is up in the air (pardon the pun), so what you may be considering as appropriate for your equipment to be "compliant" with any airline initiatives relating to voltages and/or connectors is probably based on premature and potentially inaccurate information.

The three proposals before the AEEC/CEI Subcommittee will, in a significant way, shape the future of mobile computing travel. Right now, there is no guarantee that 15 V dc is the approved or even recommended voltage for the in-seat power port. It could wind up being 20 V, 18 V , 12 V, or other voltages within the "normal" laptop range, or it may be 28 V.

Your Opinion Counts
The laptop industry didn't get much of a chance to vote on the ISPSS technologies that are already in place. You however, still have a chance to voice your opinion about the next generation of systems.

This month, the AEEC committee will be voting on the voltage and connector issues. The day before the official vote, attendees of the Portable By Design Conference, in Santa Clara, Calif., will have the opportunity to participate in a "straw poll" on the same issues. To be held Wednesday, Feb. 11, between 2:00 and 5:00 p.m. at the battery-management session, your inputs will be hand-carried to the AEEC meeting for review. Electronic Design will report on the outcome of the vote.

A working group within the AEEC/CEI Subcommittee has spent some three months gathering information from a variety of sources, including the laptop manufacturers. This fact-finding task force has put all of its research into a white paper, which has been distributed to the AEEC/CEI membership weeks prior to the Feb. 10-12 voting session. Copies of the white paper are available to any interested parties. To receive a copy, contact the Passenger Electronic Device Association (PEDA), 6320 Canoga Ave., Woodland Hills, CA 91367; (818) 887-3123; fax: (818) 883-5706).

PEDA was founded in response to the ongoing airline activities which involve mobile computing, so that there is a unified body of representatives from the notebook manufacturing sector, the battery industry, the adapter vendors, and other related PED industry segments.

TAGS: Mobile
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