A machine for testing aeroplane brake springs delivers faster and more consistent results than manual methods. Heath Reidy reports
Testing the springs in the braking system in the landing gear of an aeroplane is a crucial business. The springs apply and release the right amount of pressure in the brakes to stop the aircraft. But if they don’t work properly, besides causing the obvious problem of not stopping the plane quickly enough, or at all, they can overheat and lead to a whole list of operational problems within the aircraft.
It therefore doesn’t take a rocket scientist to realise that the procedure of testing such equipment is imperative to make sure the brakes meet the required specification.
Engineers at West Sussex firm Mecmesin, a designer of force and torque measurement solutions, have come up with a device that is providing British Airways with a quick and accurate way of testing its planes’ brake springs.
Lionel Fearon, product support engineer at British Airways, says the brake springs must be tested on every one of its planes’ overhauls. This involves the plane being stripped down, which usually takes place after every 3,000 landings.
The motorised Mecmesin system, called the MultiTest 5-x, helps speed up this process, allowing springs to be tested by compressing them in less than 30 seconds – much faster than with previous testing systems – by the touch of a button. Consisting of a lead screw attached to a motor that powers the machine, the MultiTest 5-x can exert forces of up to 500kg. When the motor is activated, it drives the lead screw, which automatically presses down and moves the crosshead of the machine against the spring, compressing it.
The motor can be set to different speeds, depending on what type of spring is being tested. This ranges from 0.1mm a minute through to 500mm a minute.
“It’s quick and simple, and makes things a lot easier,” says Fearon.
Before the motorised machine came along, Fearon says that brake springs were tested on a manual system. This involved the operator having to wind a handle to compress the spring, while looking for a digital reading. They then had to correlate this against another reading, before manually calculating whether the spring was within specification. It was all very complicated and, as it took five minutes to test each spring by hand, time-consuming too.
But compressing the spring this way didn’t just take time, it also didn’t give accurate results, as it is difficult to keep the compression speed consistent when it is done manually.
Declan Tierney, of Mecmesin, says: “When you operate a machine by hand you rely on the operator to have a consistent speed. By programming the speed electronically, it gives far more consistent results. By having an electric motor and digital speed, we can control the speed of the test very finely.”
Once the spring has been compressed, the system will display a pass, or fail, icon on a screen to show whether or not it has met the required specification. “There is no ambiguity,” says Fearon. “You get a clear pass or fail, and it actually indicates which parameter it failed against.” The screen also displays all of the spring’s statistics, such as its length and rate. So the operator is no longer required to do the calculations and write down the results.
Tierney says that this is important because sometimes damaged springs may contain micro cracks. These tiny cracks, which can cause the spring to fail when the plane is braking, are difficult to detect with the naked eye.
Although the machine cannot detect these cracks in a spring, it can detect if the spring is not within the designed specification. If a spring does contain micro cracks, it won’t have the correct spring rate and, as a result, will be the incorrect specification, so will fail the test.
Tierney says: “We don’t actually check by eye, or microscope, whether the spring has got a crack. But if it is within the specification then it will be accepted.”
Tierney says Mecmesin set the system up so every individual spring had its own test routine. The system is programmed to compress around 24 different types of brake springs, each with different characteristics. Each program is identified by the type of aircraft and its spring. So the operator can take the brake spring of, say, a Boeing 707, select the program for that plane, and the machine will automatically test the spring to the aircraft’s specification.
Fearon says: “When it comes to actually doing the test, you just put the spring in, go to the program you want, and press the start button. It’s as simple as that.”
© PE Publishing, 17 September 2008
