Engine TestingDr Colin Couper

September 10, 2014

Aircraft engines are an integral part of an aircraft and can account for up to 25% of the cost(current engines on an A380 account for almost 25% of total cost of the aircraft, costing $15meach). Such an integral part of the aircraft must pass rigorous testing both on and off the aircraft.Testing consists of a number of different stage much like the traditional V type process associatedwith certification of the aircraft. All of these parts have very common testing regimes and resultsfrom various testing phases all feed into each other in order to, ultimately, gain certification andalso be accepted by the airframe manufacturer. The following sections deal with each of thesetesting regimes, how they are carried out, what is involved and the aim of the testing.

1 Manufacturers TestingThis testing is performed on what is called a development engine and is (very basically)a way ofvalidating a design with a functioning engine.

The design of an engine is finalised using 3D models which are very good at modelling fluidflow (air flow) and temperature in the various sections of the engine. In some circumstances newdesigns for turbine blades or compressor rotors & stators will be fabricated and placed into anexisting engine in order to validate the design.

In order to test these designs as part of another engine or as a complete new design engine,manufacturers will place various sensors within the engine to monitor the parameters of as manyareas as possible and to correlate that with the modelled behaviour within the particular section ofengine. The enginewill be run through various operating loads and conditions in order to determinethe operating envelope of the engine and to determine safe operating limits.

Temperature of the various blades within the engine is very important as the material can expe-rience temperatures well in excess of the melting point of the alloy used to construct them (partic-ularly in the turbine section). This is usually measured using thermocouples for a number of reasons:

Range - thermocouples can be used up to 1480C compared to RTDs at 540C

Sensitivity - thermocouples can respond to a change in temperature up to 3 times faster thanan RTD

Ruggedness - Thermocouples are essentially a single piece, RTD elements must be connectedto an external copper wire

Strain is also widely measured in the an engine, particularly where the aircraft attaches to theaircraft or (in the case of test cells) test rig. Each area of the engine needs to be monitored forstress build up or deformation during operation cycles (dynamic strain). Standard strain gaugesare used with various configurations (full, half, quarter bridge) and are generally used to monitorthe condition of the test cell rig with some small number placed on locations within the engine itself.

Pressure is also widely measured within engines using pressure taps (imagine a static pressureport on an aircraft skin) connected to a pressure scanner. This type of pressure measurement doesnot interfere with the airflow through the engine and so is an an effective method of measuringpressure in an engine where airflow is an essential factor. Some discrete and analogue signals thatare used for control are monitored and fed back into the control loop.

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2 Certification TestingAlthough there are separate requirements for engine certification, the testing is by no means ex-clusive of the manufacturer or flight testing. There are a number of capability demonstrations thatmust be carried out by the engine manufacturer and may of these will have been carried out bythe manufacturer as part of the design verification of the engine. These are detailed in Part 33 of14 CFR and are briefly outlined below.

Stress analysis (14 CFR 33.62)Safety margins for the various rotors, spacers and turbines must be established - this is usuallydone by carrying out test on the individual components rather than when they are parts ofthe operating engine.

Vibration (14 CFR 33.63)Vibration from the engine must not induce stress in the rest of the aircraft structure - this ismonitored using strain gauges attached to the test rig in order to determine what loads arebeing imparted to the rest of the aircraft.

Surge and Stall characteristics (14 CFR 33.65)During normal operation of the engine (including startup, power change) a change in thethe input airs temperature or pressure should not cause a surge or stall to such an extent as tocause flameout, structural failure, over temperature or failure of the engine to recover duringany point of the operating envelope of the aircraft.

Bleed Air system (14 CFR 33.66)The bleed air system must not affect the engine- basically the hot temperature gases mustnot affect other parts of the engine. This is particularly important for engines that use bleedair for engine anti-icing.

Fuel System (14 CFR 33.67)The fuel system must have sufficient means to ensure that any particles that manage to passthrough a fuel filter do not impair the operation of the engine. The system must also be ca-pable of operating through its entire range with fuel saturated with water at 0.2 millilitres perlitre or contain a fuel heater or additive that ensure operation.

Induction system icing (14 CFR 33.68)The engine must operate throughout its entire range (even idling on the ground) when theicing system is in operation andmust not have an adverse affect on the engine performance.

Foreign Object Ingestion (14 CFR 33.77)This is also called the bird strike test and consists of launching a bird carcass at a critical area(the fan blades in the case of a turbine engine). The size, speed and rate of fire of the birddepends on the engine operation. Also detailed here is an ice test to simulate ice falling fromthe inlet cowl. Under all of these conditions the engine must maintain performance and notdo the following:

Catch Fire Burst (release anything from the engine case) Generate loads in the nacelle mounting in excess of the design load Lose the capability of being shut down Cause more than a 25% reduction in power Require the engine to be shut down within 5 minutes of ingestion time

Rain and Hail Ingestion (14 CFR 33.78)Engines must, at maximum power, ingest large hailstones without any mechanical damage,loss of thrust or require the engine to be shut down. The enginesmust also be tested accordingto the levels indicated in figure 1

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Blade Containment (14 CFR 33.94)Whilst operating at maximum r.p.m. the engine casing must contain a fan blade (in the caseof turbo fan engines) without catching fire and without failure of the engine mounting at-tachments. This test essentially destroys the engine but is an important test for certificationand design verification.

Figure 1: Illustration of rain and hail threats. Certification concentrations are detailed in AppendixB of 14 CFR Part 33

3 Flight TestASwith manufacturing testing, the flight test of an engine is used for both design verification and forairworthiness certification. Engines are typically mounted on a test aircraft (usually a Boeing 747)and various tests are carried out under dynamic conditions. These tests are usually carried out todetermine the actual loading conditions of the engine and how it affects the aircraft structure. Afull airborne control system is used, in some cases the aircraft will have a control system from theaircraft it is designed for installed in order to replicate the exact operating conditions the engine willexperience. Some of the Certification Tests that are required would need to be performed in theair as well as on ground with this data providing valuable feedback to the airframe manufacturer.These tests are performed with approx. 1000 sensors measuring approx 2500 parameters. Datafrom the engine FADEC is also fed back to the measurement system. The total test time for a singleseries of tests is typically 150 hours with each test flight taking between 5 and 6 hours.

4 Production TestingThemanufacturer of the engine (under 14 CFR part 21 F) must subject the engine to a test run whichhas to include a break-in run to determine the fuel and oil consumption, determination of powercharacteristics at maximum rated power. Also, the manufacturer must run a 5 hour operation testat maximum rates continuous power. This data is then included in the manual for the engine.

5 Noise TestingEach engine must have the noise level determined fr the full range of operation. This data is thencontained int eh type cert for the particular engine. The FAA define 4 types (or Stages) of aircraft

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Figure 2: GE Flight Test aircraft with a GEnx engine replacing one of the standard P&W JT9D engines

as follows:

Stage 1: Aircraft that have never been shown to meet any noise standards

Stage 2: Aircraft that meet the original limits as set out in 1969

Stage 3: Aircraft meet more stringent levels set out in 1977

Stage 4: Aircraft that meet more stringent standards set out in 2005 (applicable to all aircraftwith a Type Cert issued after 1st January 2006 with a maximum gross takeoff weight of 12500pounds or more)

The following noise limits apply to the various stages:

Stage 2 aircraft: Applies to aircraft regardless of the number of engines

1. For flyover: 108 EPNdB for max. weight of 600000 pounds. For each halving of the weight,the limit reduces by 5 EPNdB. For a maximum weight of 75000 pounds or less a limit of 93EPNdB applies.

2. For Lateral and approach: 104 EPNdB for max. weight of 850000 pounds or more. Again,For each halving of the weight, the limit reduces by 4 EPNdB. For a maximum weight of75000 pounds or less a limit of 102 EPNdB applies.

Stage 3 aircraft

1. Flyover For aeroplanes with more than 3 engines: 106 EPNdB for max. weight of 850000pounds or more. Again, For each halving of the weight, the limit reduces by 2 EP-NdB. For a maximum weight of 44673 pounds or less a limit of 89 EPNdB applies.

For aeroplanes with 3 engine: 104 EPNdB for max. weight of 850000 pounds or more.Again, For each halving of the weight, the limit reduces by 4 EPNdB. For a maximumweight of 63177 pounds or less a limit of 89 EPNdB applies.

For aeroplanes with fewer than 3 engine: 101 EPNdB for max. weight of 850000pounds or more. Again, For each halving of the weight, the limit reduces by 4 EP-NdB. For a maximum weight of 103250 pounds or less a limit of 89 EPNdB applies.

2. Lateral (regardless of number of engines): 103 EPNdB formax. weight of 850000 pounds ormore. Again, For each halving of theweight, the limit reduces by 4 EPNdB. For amaximumweight of 77200 pounds or less a limit of 94 EPNdB applies.

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3. Approach(regardless of number of engines): 105 EPNdB formax. weight of 882000poundsor more. Again, For each halving of the weight, the limit reduces by 4 EPNdB. For a max-imum weight of 77200 pounds or less a limit of 98 EPNdB applies.

Stage 4:

1. Flyover For aeroplanes with 2 engines or less: 101 EPNdB for max. weight of 385000 kg ormore. Again, For each halving of the weight, the limit reduces by 4 EPNdB until 89EPNdB after which it is constant.

For aeroplaneswith 3 engine: 104 EPNdB formax. weight of 385000 kg ormore. Again,For each halving of theweight, the limit reduces by 4 EPNdB until 89 EPNdB after whichit is constant.

For aeroplanes with 4 engines or more: 106 EPNdB for max. weight of 385000 kg ormore. Again, For each halving of the weight, the limit reduces by 4 EPNdB until 89EPNdB after which it is constant.

2. Lateral (regardless of number of engines): 103 EPNdB formax. weight of 400000 kgpoundsor more. Again, For each halving of the weight, the limit reduces by 4 EPNdB. For a max-imum weight of 35000 kg or less a limit of 94 EPNdB applies.

3. Approach(regardless of number of engines): 105 EPNdB formax. weight of 280000poundsor more. The limit decreases linearly with the logarithm of the mass down to 98 EPNdB at35000 kg.

Stage 4 figures are quoted in kg as they are defined by ICAO (International Civil Aviation Organ-isation) whereas the other 3 stages are taken from the FAA regulations (14 CFR part 36). EPNdB isthe unit used to measure Effective Perceived Noise Level (EPNL) which is the value of the Perceivednoise level adjusted for spectral irregularities and the duration of the noise. Figure 3 shows the al-lowable noise levels for Stage 2 and Stage 3 aircraft at various take-off weights. Also shown on thegraph are where current in-operation aircraft sit in terms of noise. For these tests, microphones are

Figure 3: Graph of noise levels for Stage 2 and Stage 3 aircraft from 2004 (before Stage 4 cameinto operation)

used and are placed at specific locations both on the ground with respect to the flight path ofthe aircraft. This data is then adjusted and the EPNL is determined. Figures for various aircraft areavailable from both the FAA and EASA. Figure 4 shows a diagram of measurement locations for anairport. Manufacturers would also be measuring the noise at these location in order to allow theircustomers to operate aircraft at airports.

The FAA states that maximum day-night level of 65dB is incompatible with residential communi-ties - any areas that do experience these levels are eligible for compensation. Whilst modern high

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Figure 4: Diagram of measurement points used by an airport to monitor aircraft noise

bypass turbofan engines are quieter than older turbojets and low bypass fans, there is still a push todevelop quieter and quieter engines. Some airports will not allow older aircraft with older, louderengines to operate in and out of the airport.

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