There is a saying in the nation’s capital that "money is policy." Which is to say, ignore all the declaratory political rhetoric and look to the hard numbers in the government budget.
The analog for aviation is that weight is policy. Listen to the fervent commitments to safety–then look to the weight devoted to safety-enhancing features.
A comparison of two systems illustrates the disconnect between pronouncement and poundage–in-flight entertainment (IFE) systems and fuel tank safety systems. They have been discussed separately in the May and June 2001 issues of Avionics Magazine (pages 52 and 78, respectively). Now it is time to link them into a single discussion about priorities.
Consider fuel tank safety systems first, for as this column is being written, the concern is at a high level of sweaty-palmed anxiety. In less than a 90-day period, three emergency airworthiness directives (ADs) were issued last summer and early fall concerning overheated pumps in the center wing tanks of Boeing jets. The great fear was that faulty pumps generating heat sources in the range of 1,100 degrees F would trigger another deadly fuel tank explosion of the type that has destroyed three aircraft in an 11-year period, including TWA Flight 800, which crashed in1996, killing all 230 aboard.
In the aftermath of that crash, the National Transportation Safety Board (NTSB) called for eliminating flammable vapors in fuel tanks. The reigning view in the industry was that a renewed effort to eliminate all potential ignition sources would assure safety, obviating the need for a system to inert tank vapors.
The hunt for ignition sources has proven to be analogous to the "whack-a-mole" arcade game. In this game, the player used a club to beat down plastic moles that pop up randomly out of holes in the playing board. The latest spate of emergency ADs might be described as whacks at hazardous ignition sources that keep popping up.
With almost nil notice, Federal Aviation Administration (FAA) officials called reporters to the agency’s Technical Center in Atlantic City, N.J., for a Dec. 12, 2002, demonstration of an inerting system that surmounted previous objections over weight, complexity and cost (see photo).
The system of canisters, containing membranes to separate oxygen and produce "nitrogen enriched" air to inert the empty space in fuel tanks, was a stripped-down version of a system Boeing had patented in 1983 but never deployed in commercial airliners. No regulatory requirement existed to do so, and the system weighed upwards of 1,800 pounds (816 kg). Nonetheless, if it had been installed, 239 people killed in fuel tank explosions from May 1990 through March 2001 might not have died.
In both the 1983 and 2002 systems, the membranes consist of polymeric fibers that separate air into a nitrogen-rich stream. Each fiber is about the diameter of a human hair. Air flowing into the canister encounters the inlet ends of millions of these closely packed fibers. It can escape only by flowing longitudinally through the fibers. Oxygen diffuses through the fiber walls. Nitrogen-enriched air is pumped to the fuel tanks and the oxygen is dumped as effluent.
In its patent application, Boeing claimed that the void space in fuel tanks would contain no more than 9 percent oxygen–too "lean" for an explosion–and that the system’s electronic controller would keep the tanks inerted for all phases of flight–from takeoff through climb, cruise, descent and taxi to the arrival gate. (Two of the three fuel tank explosions occurring in the years since the patent was granted have taken place on the ground). The greatest demand for nitrogen-enriched air is placed on the inerting system during descent. In engineering parlance, the descent demand is the "corner design point."
Muster a sufficient number of low-flow-rate canisters, and the peak demand could be met. But the resulting system would weigh some 1,800 pounds (816 kg). In their patent application, Boeing engineers presented a more sophisticated approach–store nitrogen-enriched air during the time of low demand in a pressurized bottle. Meter it out during descent, when the membranes cannot keep up with the high-flow-rate demand.
A dramatic decrement in system size and weight was achieved. By how much? The patent did not say, but the result was a system capable of doing more than inerting the fuel tanks. "Since the high-pressure nitrogen-enriched air can be used as a fire suppressant, the fire protection systems aboard an aircraft can be simplified," according to the patent application.
Boeing engineers envisioned a dual-use system in which the inerting gas for the fuel tanks also could be employed as a fire extinguishing gas, diverted through an arrangement of valves to protect dry bays, wheel wells and engine nacelles. Not mentioned in the patent was another candidate, the electronics and equipment (EE) bay, an area high in potential for electrical fires and presently unprotected.
With the use of lightweight materials and electronics available today, it seems fair to hazard that this multipurpose protection could be provided in a B747-size airplane for an installed weight of, say, 1,200 to 1,500 pounds (544 to 680 kg).
The lightweight system proposed by the FAA weighs much less and does less. With an estimated installed weight of 160 pounds (72.6 kg) in a B747, the FAA-announced system inerts only the center tank, not the wing tanks. It inerts to only a 12 percent oxygen concentration–perhaps not lean enough to prevent vapor explosions below 10,000 feet. The system does not provide for inerting during descent and supplies no secondary benefit in fire protection. Indeed, the FAA seems prepared to relax the inerting system performance standards outlined in its own advisory circular (AC No. 25.981-2 of April 18, 2001).
Safety vs. IFE
If this least-weight and least-cost inerting system is now the leading candidate for installation, it may be useful to point to the weight of in-flight entertainment systems. There has been no raging debate in the industry about whether the value of IFE systems in attracting passengers is worth their installed weight. Hold two inerting system weights in mind: 1,800 pounds for the all-up Boeing system proposed in 1983 (with no credit for any weight paring) and 160 pounds for the "inerting lite" concept in 2002.
An IFE system with video screens at every seat weighs collectively between 1,700 and 2,000 pounds (771 and 907 kg) on the B767-300. On the B777-200, it weights 2,800 pounds (1,270 kg). On the B747-400, such a system weighs 3,000 pounds (1,360 kg).
In other words, these state-of-the-art systems weigh the same or substantially more than Boeing’s earlier inerting concept. They are voracious consumers of electrical power, which was one of the sticking points raised during debates over the power needs of on-board inerting systems. Moreover, the IFE systems involve a whole panoply of electrical and wiring systems that impose new maintenance and trouble-shooting burdens, reflecting similar concerns that were raised by industry officials in numerous post-TWA 800 debates over the viability of inerting systems. None of the power, weight or reliability concerns were "show stoppers" for IFE systems. But they were for inerting systems until the lightweight, minimalist system now propounded came along.
Thus, the IFE on the B747-400 weighs an average of 7 pounds (3.17 kg) per each of the 416 passengers, and the partial-protection inerting system weighs about 6 oz. (170 grams) per passenger. One is left with a disturbing question of priorities.
Is it preferable to keep the passengers entertained or to keep them safe? When all is said and done about this tale of entertainment and inerting systems, the weight budget is a statement of de facto policy.
David Evans may be reached by e-mail at email@example.com.