Aircraft improvement programme at Cranfield Aerospace

Citation

(2002), "Aircraft improvement programme at Cranfield Aerospace", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 4. https://doi.org/10.1108/aeat.2002.12774daf.005

Aircraft improvement programme at Cranfield Aerospace

Keywords: Aircraft, Technology, UK

Cranfield Aerospace, one of the UK's leading aerospace solutions providers, has purchased three-dimensional measurement arm technology from FARO UK to play a major part in a world-wide project to allow greater numbers of civil aircraft to fly in the available airspace.

The new Reduced Vertical Separation Minima (RVSM) requires aeroplanes flying at between 29000 and 41000 ft in altitude to be separated by a minimum of 1000 ft. This halves the previous 2000 ft requirement and clearly places a major onus on each aircraft's altitude sensing equipment to deliver the highest possible levels of accuracy and so avoid potential collisions.

The RVSM requirement is being automatically built in to all new-aircraft being manufactured, but aircraft already in service are required to undergo rigorous testing to ensure their on-board equipment is up to the required standard, particularly when altitude readings gleaned from external sensing equipment on the aircraft are passed into the air data computer, which controls the automatic pilot. This is of course vital to ensure that the aircraft does not deviate when programmed to fly a specific course at a prescribed altitude.

The task of preparing a number of civil aircraft to meet the new requirements is being undertaken by Cranfield Aerospace on behalf of a wide variety of clients.

Project manager Peter Howarth explained, "The on-board altitude measuring equipment on most civil aircraft consists of a metallic plate on the aircraft nose, positioned at a right angle to the airflow to eliminate any dynamic effects of the airflow, with a number of holes to sense the resulting pressure. This air pressure is conveyed down pipes into the air data computer, which converts this 'static pressure' into a signal which is then passed on to the altimeter on the pilot's control panel.

"Each plate on each aircraft is of course different, and a method to scan the plate and the surrounding area on the fuselage was required. Clearly any peeling paint, cracks or dents could have a major impact on the local airflow and thus on the final altitude reading and so its accuracy needs to be carefully checked. We looked into different methods of scanning the surface and it was evident this would not be possible with traditional measuring methods".

After a trial field visit Cranfield Aerospace discovered that a FaroArm was the solution to their problem. With the arm mounted on a tripod, it could be taken up to the aircraft and used to scan around the altimeter plate. Using Faro's CAD based CAM2 software, the scan file could then be saved and outputted as a CAD file.

The accuracy of the equipment is normally verified through a flight test, where a tube with a conical end and a series of pressure tappings is trailed behind the aircraft to give a true reading of the pressure at each height. Back on the ground, this is compared with the readings achieved from the altimeter to determine the error being created through the existing equipment. If it is beyond the acceptable Civil Aviation Authority tolerance of 200 feet, Cranfield will recommend enhancements to the system or even replacement of components to bring it to the standard required. If it is acceptable, the aircraft can be certified as safe to fly for the next 12 months. Either way, an error correction curve is then programmed into the air data computer to ensure that the pilot receives altitude readings which are as accurate as possible over a range of speeds and altitudes.

The main issue is that flight testing is both time-consuming and highly expensive, particularly as each aircraft is required to be retested each year. Cranfield maintained that if in 12 months time, it was able to prove that the profile of the plate and surrounding area of the fuselage were identical or very similar to the original readings, then the error expected from the equipment could be inferred to be the same. This would mean that the expensive flight test to determine the error would not be required and the aircraft could be recertified for another year.

Peter Howarth continued, "In order for us to prove to the CAA that this technique was valid, we had to convince them of the accuracy of the measurement system. The Sterling FaroArm equipped with CAM2 measurement software can measure to ±0.102mm in accuracy and is really the only accurate way of measuring the surfaces we have to measure, which are three-dimensional, irregularly curved and contain the plate with its holes.

"Scanning of the aircraft profile with the FaroArm takes less than a day, is cost- effective, repeatable and highly accurate, requiring minimal operator training. This compares very favourably with the flight test, which takes about as long but is far more expensive in terms of pilot time and fuel costs.

"The portability of the FaroArm allows the measurements to be taken on the customer's premises, enabling Cranfield Aerospace to offer a unique service to its customers.

"Following approval of our RVSM package by the CAA, we can then make any modifications needed to the aircraft and have it ready for use within a week, so the work can be arranged to fit in with routine servicing or maintenance."

Another potential advantage of this method of measurement is for 'group approval' of larger numbers of aircraft made by an individual manufacturer, continued Howarth.

"If the nose profiles of four or five of the same model of aircraft when measured with the FaroArm are found to be within an agreed tolerance prior to flight testing, then the company could apply for group approval for RVSM and thus negate the need for subsequent flight tests altogether."

Details available from: Faro UK. Tel: +44 (0) 2476 236151, Fax: +44 (0) 2476 236150, e-mail: Kleemans@faroeurope.com Website: www.faro.com