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:
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:
Test rigs, respectively runs of aeroengines, can have deteriorating consequences (Fig. "Problems at aeroengine test rigs").
Other volumes of this book edition deal with such
problems. Hints at these can be found in the
following summary (order is no evaluation!):
What is not object of theis chapter:
Fig. "Problems at aeroengine test rigs" (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.
„A“ Particles 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:
„B” Particles 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).
„C“ Foreign 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.
„D” Supply 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 (Fig. "Failures by water in fuel").
„E“ Failures 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).
„F” Problems 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 Fig. "Contamination in electric connectors"), can produce faulty data and influence adjustments at
the aeroengine.
„G“ Vibration 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 (Fig. "Vibrations caused by the test rig").
Fig. "Disk fracture during test run" (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:
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.
Fig. "Risk of high pressure turbine test" (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:
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 (Fig. "Disk fracture during test run"). 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.
Fig. "Vibrations caused by the test rig": 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)