Emerald Group Publishing Limited
Copyright © 2000, MCB UP Limited
Tests on aircraft emergency slides
Tests on aircraft emergency slides
Keywords: Tektronix, Oscilloscopes, Flight safety
A Tektronix digital oscilloscope is playing a key part in tests designed to ensure the reliable deployment of the emergency passenger escape slides used on modern airliners (Plate 3).
The TDS210 instrument, supplied by Tektronix distributor Arrow Catalogue, is being used by Stansted Airport based AEM Limited to monitor critical performance characteristics of the discharge valves used to inflate the slides.
Plate 3 Inflated and uninflated passenger escape slides at AEM's Stansted headquarters
AEM is a specialist aviation service supplier with a division responsible for overhauling and servicing safety equipment. Testing emergency slides is an important part of its operation, and one in which 100 per cent accuracy and reliability are essential.
When operated - normally by a pull cord - the slide has to inflate in a controlled manner. If inflation occurs too quickly, it can damage the slide or cause it to inflate within the aircraft and block the escape route. Hence the gas discharge valve has to release the pressurised gas over a set period of time - typically around four seconds allowing the slide to unfold smoothly and assume its final shape without any discontinuities.
A wide variety of slides are used on different types of aircraft, and it is important for companies like AFM to build up a "library" of the ideal characteristic profiles of each type. In fact, CAA/FAA regulations require records of such tests to be kept as hard copy and filed for at least five years.
Previously, AEM achieved this by linking a chart recorder to a pressure transducer. The test engineer would start the recorder when the pull cord was activated, and hope that the discharge plot would be completed within the recorder's sweep time.
This operation involved a number of potential problems, largely because there was no guarantee that recording would happen reliably during the critical period of discharge. In addition, amplitude variations could cause the pen to jump off the paper. Interpretation of the results was also time-consuming, requiring measurements of the chart readout and calculations on the resulting figures.
When the existing chart recorder was nearing the end of its life, AEM approached Arrow Catalogue for a replacement. However, Arrow's Manager for Instruments and Tools, Robin Rayner, pointed out to them that they could carry out the job more efficiently, and at lower cost, by using a digital oscilloscope linked with a printer via its built-in digital interface. Because Arrow's catalogue contains over 1,200 different test instruments including 64 oscilloscope models, the company is thought to be well placed to give independent advice of this type.
According to Tektronix its Tektronix TDS210 was chosen because it offered the necessary performance not only to monitor the gas discharge but also to trigger automatically when the pull cord was operated - and to measure the pull force and automatically plot it on the same curve. The instrument's compact size and light weight were added bonuses, allowing it to be easily fitted into the existing test bench set-up. As a final benefit, the total cost of the oscilloscope and printer combined was less than half that of the replacement chart recorder.
The instrument is used in conjunction with a Druck pressure transducer and a MecMesin portable force indicator. The built-in signal processing within the oscilloscope converts the analogue signals from the transducers into direct readings in engineering units on the LCD screen and printer. Normally, the oscilloscope is arranged to trigger when the pull force reaches a certain value (typically 10lbs), so that it captures the entire discharge process with no wasted time at the start.
The oscilloscope and printer are set up to produce a standard plot at 10 bar/cm, compatible with the reporting format previously used and capable of being pasted directly into the final test reports.
Comment's Arrow's Robin Rayner:
The engineers at AEM are highly delighted with the Tektronix oscilloscope, which is allowing them to run their tests faster, more easily and more efficiently at a fraction of the price. They have already expressed an interest in buying further Tektronix products as a result.