24:24

24. Problems in connection with test rigs

Aeroengines undergo for different reasons test runs and certification/approval runs:

Acting in the scope of maintenance, this takes place in the as mounted condition. At airliners on the wing, at fighter aircrafts and helicopters in the airframe/fuselage. The runs serve for:

  • Review of problems, search for failures.
  • Checking (e.g., sealing runs).


After overhaul respectively repair, test and certification runs are carried out on special test rigs/beds, fixed mounted test rigs in buildings. In the military sector free standing field test rigs in buildings and outdoor, so called field test rigs (Lit. 24-3) are used.
The runs are a matter of:

  • Function check and proofs.
  • Vibration tests.
  • Verification of power and efficiency (fuel consumption, thrust).
  • Operation behavior like at start and surge margin.


Test rigs, respectively runs of aeroengines, can have deteriorating consequences (Ill. 24-1). Other volumes of this book edition deal with such problems. Hints at these can be found in the following summary (order is no evaluation!):

  • With the air ingested deteriorating foreign objects and media (volume 1, Ill. 5.5-1; volume 3,Ill. 12.4-14 ; volume 4, Ill. 16.2.2.3-2.1 and Ill. 16.2.2.3-2.2).To this belongs also the icing of the test rig. With this the danger of deteriorating ice ingestion exists (volume 1, Ill. 5.1.4-7).
  • Contaminations of the oil system and fuel system at the testrig side (Ill. 22.2.2-12.1).
  • Deterioration of the aeroengine through incorrect operation (volume 3, Ill. 11.2.1.2-7).
  • Problems in connection with measurement readings and/or its analysis.
  • Unnoticed faults and failures through unsuitable test parameters (e.g., during seal runs, Ill. 19.2-2.2 and Ill. 19.2-3.1).
  • Catastrophic failures through failing of components already damaged during service. For example investigation of vibrations with a precracked rotor disk as cause (Ill. 24-2 and Ill. 24-3).
  • Failures in connection with unsuitable, respectively problematic test equipment, e.g. lackingor not fitting bellmouth.
    Irregular power demand from accessory drive respectively power extractiion (Ill. 24-4).
    Contaminated intake area, flaking of paint, rust (volume 4, Ill. 11.2.3.2-2).
  • Blocking/clogging of the intake, e.g., by icing or a sucked in piece of cardboard or a cleaningrag.
  • Blocking/clogging of the exhaust duct, e.g., by a loose piece of sheet metal.
  • Foreign objects from the intake area/duct, e.g., fatigue fragments of sheet metal, pins of blindrivets.


What is not object of theis chapter:

  • Design and assembly/realization of the test rig.
  • Test and proof of the testrig.
  • Certification and approval by the OEM and/or authorities.

Ill. 24-1 (Lit. 24-1): During testruns on wing or at test rigs a multitide of problems can arise.
In the following deteriorations of the aeroengine by influences from the test rig are discussed.

AParticles from the environment: The experience shows, that many deteriorating particles/media can be ingested up to long distances (kilometer range). These can act deteriorating in different ways:

  • Decrease of the compressor efficiency and the surge margin. To these belong particles which stick at the compressor blading. They rise the roughness and/or change the profile. Typical example are paint mists (volume 1, Ill. 5.5-1).
  • Clogging/blocking of the cooling air ducts vof hot parts (volume 3, Ill. 12.5-4).
  • Hot gas corrosion in the hot parts. To these belong fertilizer dust (volume 1, Ill. 5.5-1) and dust of building sites.
  • Corrosion in compressor and turbine, also at disks by aggressive gases. This especially is true for accidentally air contaminations from galvanic processes (plating, etching, cleaning; Volume 4, Ill. 16.2.2.3-2.1 and Ill. 16.2.2.3-2.2). Therefore galvanic facilities, respectively its exhaust pipes, near test rigs are always suspect at such failures and should be checked.


BParticles from the test rig itself: During running aeroengine, heavy pressure fluctuations occur, which trigger vibrations of installations, especially of the intake air duct. Rust of vibrating metal sheet walls or flaking paint/coatings can clogg/block cooled hot parts of the aeroengine.The blading of the high pressure turbine is especially endangered (volume 3, Ill. 11.2.3.2-2). So the lifetime of these expensive parts can be shortened dramatically (volume 3, Ill. 12.5-4).

CForeign objects from the test rig: Typical are forgotten equipment objects of the personnel. To these count earmuffs (volume1, Ill. 5.2.1.3-1), tools and auxilary material like cleaning rags. Foreign objects can also origin from the test rig itself. For example at the intake of the aeroengine ice (volume 1, Ill. 5.1.4-3), or at steel mesh guards of the test rig icicles can form (volume 1, Ill. 5.1.4-7). This means a high failure potential for the compressor blading.

DSupply of oil and fuel: Here especically contaminations are to mention. So in one case wire bristles of a left cleaning brush in the central oil tank entered into the oil system. In an other case, water and sulfur contaminated fuel was suck. This lead during test runs to extensive failures at axial piston pumps (Ill. 22.2.2-4).

EFailures in the exhaust region of the test rig: Constrictions of the exhaust channel/duct by deformed and/or separated/loosened gas guiding sheets, can trigger a back pressure in the hot gas. This can cause outside, at the aeroengine overheating damages. Triggers a high intake temperature compressor surging, deteriorations of the aeroengine must be expected (e.g., rub down of the seals).

FProblems with sensors of the aeroengine and the test rig: For example, such a situation can emerge during icing of the pressure metering probe in the intake of the aeroengine (volume 1, Ill. 4.2-4). Problems with the connection of the probes/sensors and the data transfer e.g., corrosion or wear of the contacts (Ill. 19.2.1-1 and Ill. 19.2.1-2), can produce faulty data and influence adjustments at the aeroengine.

GVibration exitations from the test rig: Power extractions can introduce dangerous torsion vibrations at aeroengines. This danger especially exists at angeled output shafts (e.g., cardan joints), whose hinges trigger circumferential accelerations (Ill. 24-4).

Ill. 24-2 (Lit. 24-4 up to Lit. 24-8): The maintenance personnel carried out with left aeroengine a `ground run'. The cause of a worse power delivery observed by the pilot during climb had to be cleared. It showed as 2% too low for the low pressure rotation speed. The test run included an acceleration of the rotor speed up to maximum thrust (high-power engine run-up). Thereby several fast movements of the power lever from idle up to maximum thrust and again to idle took place. At the second down run from maximum thrust at about 95% rotation speed the maintenance personnel in the cockpit heared a loud explosion.
As special problem emerged, that the maintenance personnel usually switchs off the cockpit voice recorder (CVR) during ground runs. So, important data and findings about the behaviour of the aeroengine, lack.

An inspection of the aeroengine and the airpline showed:
Obviously the disk of the 1st stage high pressure turbine had fractured (sketch in the middle). It ruptured the aeroengine in two halfs. These at a time adhered free at the front respectively rear attachement. The HP disk had been ruptured from the shaft and was missing at all. Concered have been 3 equal segments and a fourth smaller fragment. One of the segments had hit the ground. Then it separated bearing structures of the fuselage (sketch above) and penetrated half into the exhaust tube of the other aeroengine (sketch below right). A second segment was found inside the fuselage. The third lay in a distance of about 800 m. It had crossed two runways and several taxiways which have been operated. The smallest piece stuck in the pylon of the failed aeroengine.

Investigation findings: The disk is from a forging of Inconel 718. It failed by a radial crack from a fir-tree groove (sketch below left) to the hub bore. The cyclic incipient crack (LCF) took place at the rear edge of a groove bottom. It started from a little notch. After it reached an axial and radial length of about18 mm, The fracture of the disk occurred. Two further grooves also showed cracks which started at small notches. The surfaces of the grooves have been shot peened as specified.
Conclusions obout the load of the disk: The low cyclic operation load at the rear edge of the disk grooves lays obviously at the limit of the material strength. So already a small flaw, respectively notch, can trigger a crack.

History: The failed disk had about 48 000 operation hours with about 9 000 start-stop-cycles (cycles since new = CSN). acceptable are only 15 000 CSN. Before the accident with about 5 000 hours and about 1 000 cycles the aeroengine was overhauled. The penetrant inspection obviously was at this time without indication. A parallel case already emerged about 5 years before at an other aeroengine of the same operator. Also during a ground run with full thrust (Lit. 24-4). Here also the second aeroengine and the fuselage had been heavy damaged. A further case occurred during climb about 4 years before at an other operator. Above this several cases with crack formation in the fir-tree grooves emerged (volume 4, Ill. 16.2.1.6-15).

Measures:

  • Already after the first case, the penetrant inspection was improved by a previous etching. Later a double etching was introduced.
  • To this a eddx current testing was added. This was `tightened' after the current case. continued page 24-7
  • All disks with reworked groove bottoms must be inspected before reaching 1675 cycles. This inspection must be repeated in correspondent intervals.
  • The geometry of the rear edges of the grooves was specified for rework. Obviously it frequently deviates from the drawing requirements.
  • Use of a eddy current test, adapted from the OEM for this case.



Comment: The cyclic incipient cracks at the rear side of the disk grooves can be explained with high thermal stresses. These add to the high tangential stresses from the centritugal forces. The thermal tension stresses may be in connection with the intense cooling of the rear grove edge. This can be explained by the passing cooling air for the turbine blades (sketch below left).

Note: To secure important data from the operation/flight before, before the ground run the CVR must be exchanged. The maintenance personnel should assure itself, that also such data of the ground run will be secured.

Note: Test runs can also mean for the personnel an increased risk. This is especally true for the case of a fragment exit, even if it is extremely seldom. Therefore oneself should if possible not stay besides the aeroengine during the test phase in the upper power range.

Ill. 24-3 (Lit. 24-4): At the turbine disks, especially during instationary processes like start or acceleration, develop locally high thermal tension stresses additional to the centrifugal forces. Is the wheel respectively disk already weakened, for example by a crack, a failure with fragment exit can happen. The following symptoms can give first hints at a coming dangerous component failing, but are not necessarily to expect in every case:

  • Vibrations, which can be allocated a main shaft system.
  • Unusual grinding/rubbing noises during turning by hand.
  • Dull acceleration behaviour of the gas producer, which can be assigned the fuel supply.

Basically additional tension stresses must be expected in the colder zones of a component. These are in the stress equilibrium forced expanded in the hotter regions (volume 3, Ill. 12.6.2-2). Reverse, thereby the surrounding hotter areas are released by compressin stresses.

The upper sketch shows a smaller aeroengine at wich the disk of the 1st turbine stage fractured during start and run up after stand still over a longer time period. Before the aeroengine showed no peculiarities during cyclic loads at the test rig. The fracture started fom an already existing fatigue crack at the hub bore (sketch below left). The test personnel was by chance not inside the test rig room. Because the hub region, especially the bore of the rotor disk is always highly loaded by centrifugal forces, intense cooling can produce locally high tension stresses. In the case shown in the sketch below right, a crack which lead to the disk fracture, started at the disk groove for the blade (Ill. 24-2). Probably especially the ege at the rear side of the disk is intensely cooled. Here enters the cooling air of the turbine blades. With this, the tension stress level is in the full load range especially high.

Ill. 24-4: Vibrations initiated into a gear can be transferred through gear wheels and lead at other locations to a dynamic overload. In the case on hand, during a certification run at the test rig, in the rated break point of the driving shaft from the fuel control unit, fatigue fractures occurred (sketch below). An investigation showed, that the test rig was modernized shortly before. Thereby, differnt to formerly, the power extraction of the accessory devices was simulated with a cardan/universal joint (sketch above). Measurements showed, that the angled cardan joints produced non-uniformities of the rotatation movement. These excited torsion vibrations, where transferred through several pairs of gear wheels up to the shaft of the control unit.

Note: Couplings like cardan shafts with nonuniform angular speed should only be used at test rigs with the approval of the OEM.

References

24-1 E.Traeger, „Aircraft Gas Turbine Engine Technology, Second Edition”, Verlag : Glencoe/McGraw-Hill 1994, ISBN 0-07-065158-2, Pages 352-366. (Buch)

24-2 V.R.Donally, „Whole Building Design Guide (WBDG), Aviation Facilities“, www.wdbg.org, Page 2-4. (4317.3)

24-3 „Prüfstände für luftatmende Luftfahrtantriebe”, www.bwb.org, Seite1 und 2 . (4317.5)

24-4 J.Hall, National Transportation Safety Board (NTSB), Safety Recommendation, „Uncontained Failure, American Airlines Boeing 767“, December 12, 2000, Page 1-8. (4480.4)

24-5 M.V.Rosenker, National Transportation Safety Board (NTSB), Safety Recomnmendation, „Uncontained Failure, American Airlines Boeing 767”, August 28, 2006, Page 1-11. (4480.1)

24-6 G.Norris, „Pictures: GE investigates cause of AA 767 uncontained failure“, Zeitschrift „Flight International”, 6/06/2006, www.freepublic.com, Page 1-11. (4480.5)

24-7 „Engine Breakup, Anatomy of an Engine Failure“, www.amtonline.com, April 10th, 2007, Page 1-3. (4480.3)

24-8 Airworthiness Directive (AD), FAA Docket No. 2004-NE-05-AD; Amendment 39-14706, AD 2006-16-06, „General Electric Company (GE) CF6-80 Series Turbofan Engines”, , Page 1-6. (4480.6)

© 2021 ITTM & Axel Rossmann
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