Study on Crack Leakage of Aircraft Hydraulic Pipeline

Zhao Jinfang

1,*

, Li Chong

1,2

, Sun Dan

, Fu Xiaopu

and Tian Linhui

Civil Aviation College, Shenyang Aerospace University, Shenyang, China

General Assembly Workshop, Shijiazhuang Haishan Industrial Development Corporation, Shijiazhuang, China

Keywords: Aircraft Hydraulic Pipeline, Crack Leakage, Maintenance Strategy.

Abstract: The problem of aircraft hydraulic pipe is one of the frequent problems in the daily maintenance process. After

a long flight, it will gradually expose structural scratches, pipe extrusion, longitudinal cracks, oil leakage,

blasting, etc. Based on the big data of the quality report of a single flight, this paper studied the main causes

of cracks at the root of the flat nozzle, and proposes targeted improvement measures from the structural layout,

operation mode, typical leakage. ANSYS simulation shows that the root of the flat nozzle of the guide pipe

is at the rear of the fuselage under different frequencies and forces. The equivalent stress and elastic defor-

mation were observed by metallography analyzer and X-ray spectrum analyzer. It was concluded that under

the installation stress of the tube itself, the root of the nozzle of the flat tube would produce extrusion friction

with the tube under the condition.The tube body was resonated with the body of the body, leading to the

fracture from the outside to the inside.

1 INTRODUCTION

The aircraft hydraulic pipeline system is the main ex-

ecutive system of the aircraft to complete various ac-

tions, which provides the pilot with the control and

power assistance of the aircraft. The fuselage of the

military aircraft is covered with densely packed avia-

tion pipelines, the number of individual aircraft pipes

about 2000, about 800 straight-through joints, which

is particularly large. After a long flight, quality prob-

lems are gradually exposed. For example, hydraulic

pulsation is very harmful to the hydraulic system,

which not only degrades the dynamic characteristics

of the system, affects the service life of the compo-

nents and pipelines, but also causes resonance or res-

onance to damage the system components, even par-

alyzes the system and causes catastrophic accidents

(Cheng et al., 2011). In the past two years, there were

120 oil leakage faults fed back from the outfield of a

certain type of aircraft. The major accidents caused

by the pipeline system accounted for 38% of the total

faults. The root cracking of nozzle is one of the most

common feedback problems follewed by pipe explo-

sion.

2 RELATED WORK

2.1 Analysis of Aircraft Hydraulic

Pipeline Problems

Flared connection is the most widely used connection

method in the active aircraft hydraulic system (Chen

et al., 2021). According to the data analysis, the explo-

sion of the aircraft hydraulic system mainly occurs at

the attachment connection, the connection of the hy-

draulic pipe around the engine, the connection of the

suspension straight pipe and the three-way pipe (Quan

et al., 2020), secondly, it occurs at the dense connec-

tion of the hydraulic fuel pipe, which produces friction,

which generates friction, indentation, compression,

pulse, etc. due to the restriction of the structural space.

The main problems focus on the following aspects.

In the actual operation of the project, during the

quarterly general inspection and spot-check of the

pipe disassembly, it was found that there was stress

assembly in many pipes, and forced assembly was

carried out without qualified adjustment in the pro-

cess, which also indicated that the stress release was

not achieved during the flight (Xia et al., 2021). Due

to the restriction of space, the installation of hydraulic

system pipelines is inconvenient and prone to friction,

which is mainly manifested in severe pressure pits,

Jinfang, Z., Chong, L., Dan, S., Xiaopu, F. and Linhui, T.

Study on Crack Leakage of Aircraft Hydraulic Pipeline.

DOI: 10.5220/0011955700003536

In Proceedings of the 3rd International Symposium on Water, Ecology and Environment (ISWEE 2022), pages 187-191

ISBN: 978-989-758-639-2; ISSN: 2975-9439

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

187

friction of structural stringers, wear through and com-

pression of peripheral pipelines (Heng, 2014). The

cause of the problem comes from the deficiencies in

the aircraft design.Most of the special frequently oc-

curring pipelines are located behind the inlet with

strong vibration, and the inlet is the boundary, except

for the front part of the fuselage of the main landing

gear, they are more stable (Meng et Yang, 2022). The

leakage rate of the rear fuselage is 85% of that of the

whole fuselage, and the tube is installed near the air-

craft engine, and the working temperature is about

550°C. The flat nozzle is an integral life part of air-

craft, which is easy to produce longitudinal fatigue

crack in its horn and pipe bushing (Liu and Xao,

2016). The failure fluctuates during the training pe-

riod. The failure of the aircraft just completed the test

flight is extremely low. When the flight time is 300h-

400h, the failure rate of the aircraft is high, and the

failure rate increases with time. The cause of correc-

tion is related to failure on the one hand and stress

assembly in actual operation on the other hand.

The occurrence of problems can be described as

diverse and complex. According to the statistical

chart of oil leakage locations as shown in Figure 1,

the oil leakage at the root of the nut accounts for 59%

of all oil leakage locations. The root of the nut is the

location where the nut contacts the flat nozzle. It can

be seen that the contact between the nut and the pipe-

line is the main problem of oil leakage at the joint

with lock nut(Chen, 2021; Du et al., 2021).

Figure 1: Statistical Diagram of Oil Leakage Location.

2.2 Analysis of Crack State of Aircraft

Pipeline

2.2.1 Post Damage Caused by Aircraft

Engineering Specifications

The aircraft hydraulic pipeline can be installed in two

ways, the first is divided by system, and the second is

divided by location. The hydraulic system installed

according to the system division is assembled by a

single operator, who will adjust and control according

to the overall length; The hydraulic system installed

according to the location division is usually assem-

bled by multiple operators. For example, one operator

is responsible for the front of the air inlet and another

operator is responsible for the rear of the air inlet.

This will lead to poor stress control on the pipeline

butt joint after the hydraulic system is assembled.

Sometimes, it can not even be assembled normally,

and the pipe can only be sawed for new assembly.In

the process of pipe assembly, some aluminum alloy

pipes are affected by the manufacturing tolerances of

pipe materials and flat nozzles, as well as the manu-

facturing errors of flared taper, outer sleeve nuts and

straight joints.Therefore, the fit gap between the pipe

and the flat nozzle tube will be randomly distributed,

and there are different coaxial phenomena, once it

happens, it is an accident sign(Du et al., 2021). From

the project management mode, the first operation as-

sembly mode is to install according to the system di-

vision, the whole hydraulic system installation by the

operator single assembly, will be adjusted according

to the overall length of processing; The second is di-

vided by position, such as one operator before the in-

take port and another operator after the intake port,

after the assembly of the hydraulic system, the con-

nection of the two pipelines will result in poor control

and produce stress, sometimes resulting in stress in-

stallation. If the normal assembly can not be

achieved, the saw pipe can only be reassembled, so

most of the stress is difficult to control, complex pipe-

line network even more than 10 mm there will be a

big problem. At the same time, some aluminum alloy

pipes are affected by the manufacturing tolerance of

pipe material, flat nozzle, taper deviation of flaring,

outer nut and straight through joint and so on, the fit

gap between the pipe and the flat nozzle will be ran-

domly distributed, and the pipe and the flat mouth

tube have different axis phenomenon (Du et al.,

2021). When it happens, it is a symptom of an acci-

dent.

In the outfield flight mission, it was reported that

more pipeline cracks were generated at the root of the

flat nozzle. At the same time, it was found that there

were metal chips like foreign matters with the same

material as the pipeline between the pipeline and the

flat nozzle, mainly concentrated in the longitudinal

cracks between the pipeline and the bushing, and the

cracks between the pipeline and the bell mouth. Due

to the deformation caused by vibration, pulling and

force, the pipeline cracks often occur at the root of the

ISWEE 2022 - International Symposium on Water, Ecology and Environment

188

sleeve. Figure 2 shows the internal morphology of

high circumferential cracks.

Figure 2: High cycle crack at sleeve root.

The reason for the crack at the root of the flat noz-

zle is that there is a stress at this position, which

causes the extrusion between the pipeline and the flat

nozzle. In addition, the aircraft will generate vibration

during flight. The two factors act at the same time,

causing the abrasion of the pipeline and eventually

leading to the breakage of the pipeline.

2.2.2 The Frequent Problems of Pipelines

Leakage

After the actual assembly, the stress of the aircraft

will be released after a long period of pressure. How-

ever, if the release is not complete enough, the vibra-

tion of the aircraft and the start of the engine will

cause great vibration damage to the hydraulic pipe. If

the problem of excessive stress is encountered, the

operator will usually fine tune the angle or select the

saw tube installation. At present, L2FM pipes in

China are often found with surface defects such as in-

dentation, scratch and other problems in the process

of use, which are caused by cracks in the pipe under

the vibration environment after the aircraft has been

flying for a long time.

The pipeline for body installation is divided into

formed tubes and non formed tubes. There are strict

requirements for unformed pipes to be self calibrated.

The bending radius should not be less than 3d (d is

the diameter of the pipe). The linear distance from the

beginning of the pipe bending to the flat nozzle

should not be less than 5mm. The pipe should not be

twisted after connection. The cause of scratches is

usually that the bending radius is too small, and the

stress is formed at the root of the catheter and the

sleeve, which causes the pipe to bend directly at the

sleeve, resulting in root leakage.

2.2.3 Leakage Caused by Repair and

Maintenance

In order to improve the maintenance efficiency and

eliminate the leakage in time, the maintenance per-

sonnel usually dismantle the pipeline at the connec-

tion and then forcibly connect it. This process often

does not consider the requirement of force limitation.

Such disassembly and assembly will not only cause

transverse cracks at the pipe bell mouth, but also eas-

ily lead to sudden changes in the contact surface, re-

sulting in pressure or contact clearance, which will

lay hidden dangers for safe flight. For example，

When disassembling the two-way connecting pipe-

line, if the two ends of the fixing nut are forcefully

disassembled, the pipeline about 10mm behind the

bell mouth at the other end will be bent at 60°, and

even if it is recovered, there will be poor contact.

3 METHODOLOGY

3.1 Control Measures to Prevent

Pipeline Damage in the Later Stage

According to the theory of total quality management,

the factors affecting the damage of the pipeline are

studied, and the damage caused by vibration or wear

is controlled through feedback improvement.

3.1.1 Improve Maintenance Project

Management Mode

A typical "star" pipe after complete installation is

tested for stress compliance during installation.

(1) 70% of the faults in the hydraulic system are

caused by the oil pollution of the hydraulic system.

Therefore, it is extremely important to strengthen the

control of oil pollution and surplus in the production

process to improve the reliability of the hydraulic sys-

tem. The timing of oil filter connection shall be

strictly controlled (Zhu et al., 2015). The equipment

shall replace the oil filter in strict accordance with the

implementation standards to avoid the frequent infil-

tration of surplus materials into the system.

(2) Adjust the process to avoid multiple disassem-

bly and assembly in the later period, which may lead

to poor contact of the catheter. The pipeline shall be

strictly checked to see if there is scratch inside it and

if there is cone indentation.

(3) The important connection position of the pipe-

line more or less caused uneven stress release due to

improper adjustment in the early stage, resulting in

Study on Crack Leakage of Aircraft Hydraulic Pipeline

189

smaller gap and larger stress at the connection. The

system connection can release the whole to the opti-

mal configuration range to solve this problem. The

pulsating production line shall be established for the

installation of hydraulic pipelines to avoid the whole

machine spreading for operation as far as possible.

The accessories of the hydraulic system department

shall be taken as the starting point for treatment, and

the work rhythm shall be adjusted to improve the

quality of pipeline maintenance.

(4) The pipeline shall be protected in a timely

manner by professional binding, and the paint at the

root of the pipeline shall be repaired in a timely man-

ner to prevent corrosion of the pipeline caused by

moisture, acid and alkali substances. In addition, dust

and air shall be prevented from entering the system.

3.1.2 Ensure the Process Control of Pipeline

The pulsatile production mode can realize the trans-

formation of the position set by the system, so as to

reduce the excessive stress at the junction.

(1) Correct sizing and stress relief of the pipeline.

Due to the influence of the material of the hydraulic

pipe itself and the large bearing pressure, there is a

large stress in the hydraulic pipe during flight, which

may lead to air blasting. When installing the pipeline,

install the nut, and then gently pull it with your hand

to conduct stress free installation. Do not force instal-

lation.

(2) Strictly enforce the torque requirements of the

pipe, do not bear too much force, according to the

pipe diameter to apply a considerable force, in prac-

tice due to poor contact, want to use the way of screw

die is not right, there are also short horn, stud

scratches generated by the surplus caused by system

pollution. Each type of pipe has torque requirements,

according to the different torque to tighten.

(3) Each type of pipeline has torque requirements,

which shall be tightened in strict accordance with the

torque requirements of the pipeline, and excessive

force is not allowed. In actual operation, it is not ad-

visable to deal with the problem of poor contact by

screwing the nuts. This practice will cause the stud to

scratch and produce surplus material, which will

cause system pollution.

(4) For the pipelines near the engine, try to avoid

the hard connection mode of empty three-way and

four-way pipes. If this mode is necessary, fix the

three-way and four-way pipes on the engine body and

accurately measure the distance (Zhang et al., 2019).

(5) If the pipeline leaks, the cause shall be ana-

lyzed in a timely manner, and the troubleshooting

cannot be carried out simply by tightening. In addi-

tion, protection measures shall be taken during instal-

lation to prevent foreign matters from entering the

system and damaging the inner wall. The causes of

frequent catheter damage should be found by combin-

ing data analysis.

4 CONCLUSIONS

Pipeline failure is a system engineering, and the fail-

ure process is relatively complex, usually caused by

multiple factors. There is no obvious plastic defor-

mation in the fatigue crack of the pipeline. Without

obvious signs before fracture, it will suddenly cause

damage, and the fatigue fracture stress is very low,

often lower than the yield strength under static load.

ACKNOWLEDGEMENTS

This work was financially supported by Scientific re-

search fund project of Liaoning Provincial Depart-

ment of Education (JYT2020122); Shenyang Aero-

space University introduction talent scientific re-

search start-up fund project (20YB20).

REFERENCES

Cheng R, Wang X, Yin Z. (2011). Cause Analysis and So-

lution of L15 Higher Education Machine Hydraulic

Pulse Exceeding the Standard [J]. Trainer, (01): 46-49.

Chen D, Yang J, Li W, et al. (2021). Research on connec-

tion simulation and structural parameters of aviation

flared conduit [J]. Journal of Sichuan University of

Light Chemical Technology (Natural Science Edition),

34 (04): 25-31.

Quan L, Che S, Guo C, et al. (2020). Axial Vibration Char-

acteristics of Fluid-Structure Interaction of an Aircraft

Hydraulic Pipe Based on Modified Friction Coupling

Model[J]. Applied Sciences, 10(10): 35-48.

Xia Z, Fan X, Zhao X, et al. (2021). Simulation Analysis of

Multi factor Influence Law on Pipeline Sealing of Air-

craft Hydraulic System [J]. Aerospace Precision Man-

ufacturing Technology, 57 (03): 5-10.

Heng B. (2014). Finite element simulation analysis of con-

nectors and pipelines of aircraft hydraulic system [D].

Nanjing University of Aeronautics and Astronautics.

Meng Q, Yang G. (2022). Fault diagnosis method of hy-

draulic pipeline based on LSTM neural network model

[J]. Electromechanical Engineering, 1-9.

Liu Z, Xiao J. (2016). Failure analysis of flat nozzle crack

in high temperature zone [J]. Economic and Trade

Practice, (22): 238.

ISWEE 2022 - International Symposium on Water, Ecology and Environment

190

Chen D. (2021). Mechanical response analysis and struc-

tural optimization of aviation flared pipe joint [D].

Southwest Jiaotong University.

Du J, Sun T, Tan Y, et al.(2021). Failure analysis of circum-

ferential fatigue cracking of flared 5A02 aluminum al-

loy conduit [J]. Light alloy processing technology, 49

(04): 38-43.

Zhu Z, Tao Y, Wang X, et al.(2015) The importance of pro-

duction process is seen from the maintenance of avia-

tion hydraulic pipe [C]. Proceedings of the Seminar on

Aviation Equipment Maintenance Technology and Ap-

plication. Aviation Maintenance Engineering Branch of

China Aviation Society: China Aviation Society,: 511-

514.

Zhang F, Yuan Z, Zhang F, et al.(2019) The analysis and

estimation of vibration fatigue for pipe fitting in avia-

tion hydraulic system[J]. Engineering Failure Analysis.

Volume, 105:837-855.

Study on Crack Leakage of Aircraft Hydraulic Pipeline

191