Metrology gives a boost to aircraft engine maintenance
Exceptional measuring accuracy plays a central role during maintenance of the aircraft engines in the Lufthansa fleet. For the specialists who carry out this work, the universal length measuring machines from Mahr – the most accurate series-manufactured machines of their type available in the world – are an indispensable tool.
The aircraft engines of the global Lufthansa fleet are serviced at the Lufthansa Technik aircraft maintenance center in Hamburg. As an elementary component of an aircraft, they are subject to the highest safety requirements at Lufthansa. With good reason, as the engines are exposed to extreme stresses: the blade wheels of a turbine spin up to 16,000 times a minute and are exposed to temperatures of up to 1,600 °C in the process. Sensors are used to continuously monitor for example the exhaust gas temperature and vibrations during the flight. Maintenance measures are triggered if the measured values exceed a defined limit. In the event of wear the engines are completely taken apart in the Lufthansa Technik hangar in Hamburg. They are then cleaned and inspected, and any defective parts are replaced as required. As part of this process, every component is re-measured.
With the cooperation between Lufthansa Technik and Göttingen-based measuring technology manufacturer Mahr, two partners have joined forces who both set the highest quality standards and attach great importance to highly accurate measurements. For example, Lufthansa Technik adopts a 3-tiered measuring process when inspecting drilled holes. Firstly, a 3-line internal measuring device is used for direct measurement of the bore. This handheld measuring device is regularly calibrated with the aid of a setting ring gauge. In turn, the setting ring gauge is regularly measured on a high-precision length measuring machine – and this, finally, bases its measured values on the measurement standards of the Physikalisch-Technische Bundesanstalt (PTB) in Brunswick. The PTB is the national metrology institute and the highest technical authority for metrology in Germany.
One example: the turbine engine stator. On this heavyweight turbine component, employees at the aircraft maintenance center in Hamburg check the large number of bores to which the non-rotating turbine blades are attached. The work is performed with the aid of 3-line inside micrometers, e.g. the Micromar 44 A from Mahr. With this handy micrometer, the bore dimensions can be measured quickly and accurately, and this means that wear can be detected prompty and reliably. The parts under testing have error limits of e.g. 12 µm. For a reliable measurement, the maximum deviation of a 3-line inside micrometer is just 4 µm on a diameter of 37 mm (DIN 863-4). With a specified error limit of 4 µm, the Mahr 3-line inside micrometer is therefore perfectly suited.
In order to ensure continued compliance with this specified maximum error limit, the 3-line inside micrometer is itself recalibrated and tested to ensure that it works perfectly at yearly intervals in a climate-controlled calibrating laboratory.
Before the 3-line inside micrometer can be used, it has to be adjusted on-site to the inscription value of a setting ring gauge. This ring, which is made of solid gauge steel, has a very accurately specified diameter. Here, the dimensions correspond to DIN 2250 C and the error limits to DIN 2250. According to the requirements specified by Lufthansa Technik, the accuracy of the error limit must exceed that of the 3-line inside micrometer by a factor of 3.
Lufthansa Technik has very high demands when it comes to reliably checking the 3-line inside micrometer. At Lufthansa Technik, the error limit on the diameter of the setting ring gauges used for this task is just 1 µm (2 µm would be permitted according to the standard).
A new setting ring gauge comes with a calibration certificate, in which the diameter and the measuring uncertainty are certified. However, even these setting ring gauges are subject to a certain amount of wear and aging, both of which affect their material properties and, ultimately, their diameter. This is why the setting ring gauges need to be re-measured periodically. In order to maintain the company’s high quality standards, Lufthansa Technik performs these tests itself on a high-precision universal length measuring machine, the Precimar 828 CiM 500 series model from Mahr.
The length measuring machine is motorized, rests on a bed of granite and stands in a climate-controlled metrology lab in Hamburg. With a measuring uncertainty of 0.1 + L/1000, the measuring machine is perfectly positioned for reliable measurements within the specified error limit of 1 µm – the latest Precimar 828 CiM generation even offers a measuring uncertainty of just 0.075 + L/1500. Mahr achieves this extremely high measuring accuracy through a combination of running the components on air-lubricated bearings, generating the measuring forces with practically zero friction, strict compliance with the Abbe comparator principle, a high-end incremental distance measuring system and a CNC-controllable measuring slide and object table. Employees at Lufthansa Technik measure the setting ring gauge in 3 planes and 2 cross-sections. As the length measuring machine runs automatically, the 6 diameter measurements required for each setting gauge ring are performed very quickly and virtually independently of the operator.
In order to calibrate the setting gauge ring, the length measuring machine has also been equipped with a rotary indexing device – a component which is only offered by Mahr. The advantage of this layout is that the setting gauge ring does not need to be handled and clamped back in the measuring machine, as the rotary indexing device can rotate it quickly and accurately through 90 degrees. This offers another advantage in that it prevents the slight warming of the setting gauge ring that would otherwise occur during handling, which would slightly deform or expand the ring.
In order to ensure that every step in the quality chain is maintained, Lufthansa Technik has the 828 CiM 500 serviced and recalibrated once a year by Mahr specialists. This is done with the aid of DKD-calibrated gauge blocks, which can be traced back at Mahr to the measurement standards of the Physikalisch-Technische Bundesanstalt (PTB).
The exceptionally high level of quality assurance in the Lufthansa Technik aircraft maintenance center in Hamburg has not only convinced quality management at Lufthansa. Other international airlines are now also having their aircraft engines tested here as well. Of course, the passengers onboard the aircraft don’t usually notice any of the highly detailed work which goes on behind the scenes at the aircraft maintenance center – instead, they enjoy a problem-free and safe flight. And this is exactly what makes all the efforts
of Lufthansa Technik and Mahr worthwhile.
– Andreas Gossing, Kurt Koch, Dr. Jürgen Schweizer -
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