FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1981
1981 - 1851.PDF
FLIGHT International, 6 jane 1981 1773 -< -*' page 1767 rounded by a sealed layer of insula tion, which in turn is protected by a layer of flexible open-cell foam purged with nitrogen. The air spaces fore and aft of the tanks are purged with posi tive airflow. About 2001b of nitrogen would be required for a 5,500 mile flight. Hydrogen sensors are located at the discharge vents to detect leakage. The LH2 will be used as a heat sink for cooling engine oil, cabin air and turbine-cooling air. These uses of the cold fuel will improve range by about 8 per cent. The aircraft tanks should always contain fuel, except during major maintenance, to elimi nate the "chill-down" process required if tanks have been allowed to warm to ambient temperature. LH2 pump design has a consider able missile and space background, acknowledges Green, "but much more development is required to achieve the long life needed in transport aircraft." The engine-driven HP pump is ex pected to be "a much more serious design problem than the boost- pumps." Rapid evaporation Although LH2 evaporates rapidly on spilling, weak ignition sources can produce a flame; and extra pre cautions to avoid even the smallest electrostatic spark are required wherever air/hydrogen mixtures are likely to occur. And the coldness of the fuel can cause personnel injuries. Special gloves, clothing and non-con ductive footwear will be required, and personnel must be trained in their use. Any LH2 trapped in a closed volume can produce serious consequences, warns Green, and any system or con tainer must have some type of pres sure-relief. Hazards resulting from the entry of air into any part of the LH2 system "must be deeply ingrained into all maintenance personnel." Adds Green: "Liquid hydrogen is globally available, with many econo mic and environmental advantages." Lockheed believes that "after proper training LH2 will probably be just as safe, if not safer, in service than cur rent or other potential alternative fuels." In the early days, avionics main tenance was on the whole simple— "If you could hear it, it was working." Then, according to Bill Grice, British Airways avionics and electri cal engineering manager, frequent and expensive false removals, coupled with the difficulty of isolating avionics faults on the ground, caused airline management and pilots to regard fur ther expansion of avionics with suspi cion. The next step was the introduction of self-test procedures and mechan isms. These had limited success, usually failing to provide comprehen sive fault-diagnosis, and often increas ing failure rates. With the last of the analogue-avionics generation, recalls Grice, came Bite (Built-in-test equip ment). The idea was that avionics should be self-testing, but "this did not always work out too well". Test ing and fault-diagnosis were very much post-design considerations. "These factors may explain the past lack of success, and may give some hope for the future in the era of digi tal technology." Fortunately, says Grice, it is very much easier with digi tal devices to detect failure, and to design hardware and software so that testing and diagnosis are an integral part of the system at a very early de sign stage." With the digital computer the in puts and outputs are "time-multi plexed," and failure results in a com plete disruption of all data-flow. Monitoring to detect such a failure can be made, part of the computer's integral design. The same circuitry that performs the function can be used for the monitoring. The new self-checking and self- monitoring digital systems are based on sound engineering ideas, says Grice, and there is reasonable hope that their promise will be fulfilled. "Much remains to be explored, par ticularly in maintenance-monitoring. Avionics has a role to play in the monitoring of engines and airframe structure. Such tasks will be made easier by the continuing development of flight-data recording, and by the maintenance crew's ability to record >- >- page 1777
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
