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SUBJECT: BLEED AIR LEAK warning and trouble shooting recommendations
ATA CHAPTER: 36-22
AIRCRAFT TYPE: A320Family
APPLICABILITY: All aircrafts
REFERENCES: TFU 36.22.00.009
TFU 36.11.49.007TFU 36.11.49.008
TFU 36.00.00.002SB A320-36-1025
SB A320-36-1039SB A320-36-1043
AMM 36-22-00, Page block 001TSM 36-22-00, Page block 201
DRM, Page block 601CMM 36-11-17 and 36-11-18
Wise article EngOps-16312 (in replacement of the SIL 36-032) Wise article EngOps-16389 (in replacement of the SIL 36-036) Wise article EngOps-16473 (in replacement of the SIL 36-043) Wise article EngOps-16477 (in replacement of the SIL 36-046) FCOM PRO-NOR-SOP « After Landing »
1. PURPOSE:
The purpose of this document is to provide operators with a general review:* Of the air bleed overheat detection system fault logics,
* Of improvements today available to improve aircraft resistance to bleed air leaks,* Of preventive actions which, when implemented, will improve aircraft resistance to bleed air leaks. This document was revised with regard to the new MPD interval criteria policy.
The document was revised in the aim to provide Temperature templates Tool Part Numbers and to reflect RevisedMPD intervals.
This document is revised in the aim to update the reference of Temperature Template TK90008B replaced by08STHEB.
2. BACKGROUND:
Operators have reported difficulties in trouble shooting bleed air leaks. This resulted in long aircraft down time and operational delays.
3. DESCRIPTION:
In order to ensure an efficient trouble shooting of bleed air leak warnings, understanding of the different types of leak and different warnings is necessary. .
A. SYSTEM DESCRIPTION:
The overheat detection system is installed in order to detect all temperature increases, which could generate damage to structure or surrounding equipment and affect their reliability.
It covers locations where such leaks can occur, such as bleed air ducting, for which it consists of sensing elements.In case of a bleed air leak, the overheat detection system triggering limit will be reached only if the subject leak is significant enough to heat up the surrounding air. This overheat detection triggering limit is between 117°C and 131°C (242°F and 268°F) for all location except for the pylon, for which the overheat detection limit is between 192°C and 216°C (378°F and 421°F).
The air leak warning will be generated only if this increase of temperature occurs for at least 20 seconds.
The overheat detection system is ensured by a dual loop system for the wing leak detection and a single loop detection for the pylon and the APU ducting leak detection. For further details regarding Dual loop and Single loop system operation refer to AMM 36-22-00 page block 001.
B. DIFFERENT TYPES OF LEAK:
(1) Leaks which occur on ground or which can be reproduced on ground:
This kind of leak consists of a heavy leak, which very soon (approximately 7 seconds) after hot air is present in the aircraft ducting (APU or ENG bleed 'ON') generates a leak warning. It can result from:
* The duct itself (damaged, burst duct).* Duct connections (damaged or torn flexible joint seal, or combo seal, damaged or cracked duct flanges or loose clamps).
(2) Leaks which cannot be reproduced on ground:
'Small leaks', which are not systematically detected. For some of them, which are localized on duct connectors or flexible joint seals, aircraft movement (taxi, T/O, cruise, landing, ...) can lead to duct movement and then increase the leak rate to generate a temperature above the overheat detection triggering limit.This kind of leak can be split in two as follows:
* Leak ECAM warning
* Loop A(B) messages
The second type (Loop A(B) messages) has been found to be mainly due to a leaking connector. This connector experiences a limited leak, or starts to leak. In this case, the leak is directed on only one sensing element of the dual loop, and the leak is not large enough to heat up surrounding air to be also detected by the second element.
(3) Leaks which can be reproduced on ground under specific conditions:
(a) Operation on ground:
In some cases, leaks can occur after long operation on ground parking, with APU bleed or ECS running, or during taxi waiting for clearance.This happens when solar and tarmac radiation heats the aircraft structure, with no venting of the wing leading edge and air conditioning pack bay.
In this kind of situation, a 'small' leak, which is usually not detected, would more easily heat up overheat sensing elements to their alarm triggering point.
These ‘small’ leaks are sensitive to Outside Air Temperature fluctuations as follows:
A ‘small’ leak which would raise the bleed air ducting ambient temperature around 115 °C (239 °F) with anoutside air temperature around or lower than 15 °C (59 °F) is not detected. The same leak present with a
‘high’ outside air temperature of 35 °C (95 °F) would raise the bleed air system ambient to around 135 °C(275 °F) and therefore would be detected by the overheat detection system.
(b) Full Environmental Control System (ECS) demand:
Leak warnings may occur only during Full ECS demand, as the duct internal bleed air flow demand is increased. It can increase the leak rate of a 'small' leak to the sensing element alarm triggering point.
(4) Spurious leak warnings:
(a) When this kind of warning is generated without a pressurized bleed air system, it can be due to:
* Damaged (cut) sensing element: electrically open circuit.
* Damaged (dented or cracked) sensing element: electrically short circuit (permanent detection).
(b) When this kind of warning is generated only when hot air is present in the system, it can be due to:
* One of the two overheat sensing elements or the two overheat sensing elements, which is (are) too close to the duct, and which is (are) heated by duct heat radiation.* Degraded Overheat Sensing element, for which the triggering point would be too low.
This is the reason why in leak trouble shooting it is recommended to first ensure that the overheat detection system is fully operative.
C. ECAM WARNINGS / MAINTENANCE STATUS LINKED TO THE OVERHEAT DETECTION SYSTEM
(1) ECAM Warnings:
(a) One Loop Inoperative and One Loop Detecting:
If one loop has been seen as inoperative and the second loop enters detection status, the ECAM warning“L(R) WING LEAK” is triggered requiring the appropriate action from the crew.
(b) Two Loops Inoperative or One Loop Inoperative with Opposite BMC Faulty
This situation means that a possible leak cannot be detected. In this case an ECAM warning “L(R) WING LEAK DET FAULT” is triggered with the associated CFDS maintenance messages "L(R) Wing Loop A(B) INOP” and “BMC 1(2)”.
(2) Maintenance status:
(a) One Loop Inoperative (Open Circuit)
During the BMC test the continuity of the circuit is tested. If the continuity is broken (that is open circuit), a CFDS fault message “L(R) Wing Loop A(B) inop” is given associated to an “AIR BLEED” Maintenance Status. However, the fault can remain transparent, if it occurs after the BMC test during aircraft power-up test (or BMC test) has been carried out, but does not affect the loop detection system.
(b) Leak Detected by a Single Loop
Under these conditions, one of the loops senses an overheat but the other loop remains below this condition. An “AIR BLEED” Maintenance Status appears in conjunction with the CFDS fault message “L(R) Wing Loop A(B)”.
(3) Permanent warning:
(a) Permanent Detection (Short Circuit)
A continuous overheat detection is given by one loop. This is due to a short circuit of the system due to damage to a sensing element (dents or cracks). This condition results in an “AIR BLEED” Maintenance Status associated to the CFDS fault message “L(R) Wing Loop A(B)”.
4. AIRBUS ACTION:
In order to improve the bleed air ducting reliability (protection against bleed air leaks), several actions have been launched.These actions consist in improving the bleed air ducting and overheat detection system, to review the fleet
experience with leaks in order to assist operators to identify the main origins of bleed leaks, and finally to propose preventive actions, which could reduce the number of bleed air leak occurrences.
A- IMPROVEMENTS
(a) Flexible joint seals improvements:
Improvements consist in modification of the sealing of the flexible joint. This involves modification of the shape of the seal as following:* Before March 93: ‘O-Ring’ as per Fig 1.a.
* After March 93 ‘Vari-seal’ as per Fig 1.b.
Note that this product improvement did not result in duct PN modification ((TFU 36.11.49.007‘Wing to fuselage bleed air duct leakage’, Closed Jun-93).
(b) Repair / replacement procedures:
In addition to the above, introduction of a repair procedure to allow either flexible joint seal replacement and/or repair, has been implemented through:
* A319/A320/A321 DRM Rev Jun-97,Which lists all the duct Part Numbers with a flexible joint seal and covers pressure tests of the duct assembly, and refers to the CMM for details regarding repair and replacement.
* CMM 36-11-17 and 36-11-18 Dated Jan-98,
Which details repair and replacement procedures of these flexible joint seals.Complete duct / joint / insulation refurbishment can be accomplished by returning duct assemblies through Airbus MSC or ANA.
Note: Temporary repairs using sealant are not recommended because filling the joint will impede flexibility and may stress the duct. If the joint does flex, the sealant will tend to break away allowing further leakage. In addition, experience has confirmed that a subsequent permanent repair is not practical due to internal damage to the joint caused by the sealant.
(c) Installation modification:
In the frame of installation improvements the areas where GRIMES flexible seals are installed have been targeted for improvements.
- In fuselage FR40, GRIMES flexible joint seals (Fig 2a) have been replaced by ‘GIMBAL’ joint seals (Fig2b).
This change is identified by MOD 24200, and embodiment rank is:* For A319: MSN 546
* For A320: MSN 566* For A321: MSN 570
- For the wing area, the possibility to have GRIMES flexible joint seals (Fig 2a) replaced by ‘GIMBAL’ joint
seals (Fig 2b) has been reviewed and confirmed as possible. (TFU 36.11.49.008 refer). This change is identified by MOD 28414, and embodiment rank is:* For A319: MSN 1082
* For A320: MSN 1187* For A321: MSN 1188
In the wing area, GRIMES flexible joint seals and ‘GIMBAL’ joint seals are interchangeable.
1- Overheat sensing element installation
A minimum distance between duct insulation and sensing elements has been introduced in the AMM RevNov-97 (Fig 3 refers).
This minimum distance has been introduced in order to avoid any contact between overheat sensing elements and the bleed air ducting. This would prevent overheat detection triggered by overheat sensing elements due to thermal conductance from the ducting. This is valid especially for the hot parts of the duct, such as that duct connections, duct clamps, or valves which are fitted directly on the duct and not on the insulation cover.
This minimum distance is 12,7 mm (0.50 in.) between the element and the ducting, when the overheat sensing elements are installed directly on the ducting. In areas where the overheat sensing elements are fitted on the structure there are no specific clearance recommendations.
2- Bleed air duct seals
(a) Silicon combo seal improvements
Details regarding the different evolutions of silicon combo seals up to the introduction of the improved
ABS0737 combo seal standard are given in Wise article Engops-16389 (SIL 36-036).
Incorrect installation of subject seals can lead to early degradation of the seal material and then result in a leak.These seals should be replaced on condition. A systematic replacement of the combo seals would have to be considered only for aircraft pre ABS0737 standard (Wise article Engops-16389 refers).
(TFU 36.00.00.002 Bleed System Seal Reliability, Closed Feb-95)
(b) Silicon combo seal improvements
Investigations launched in order to improve the bleed air ducting air tightness led to the definition of the ABS1040 seals.
These seals are two parts and are composed of a filler ring and the seal itself, both of which have to be installed in the outer grove of the duct. In comparison, the ABS0737 silicon combo seal is installed in the two grooves.This new seal is made of a blended PTFE seal material.
Embodiment of this ABS1040 standard is proposed one way interchangeable by MOD 32027, andproduction embodiment rank is:
*For A319 MSN 1831 *For A320 MSN 1873 *For A321 MSN 1830.
This modification cancels the MPD requirements for preventive seal replacement as per paragraph C. (1).
The Wise article EngOps-16389 (SIL 36-036), dealing with the different bleed air duct seal evolution, hasbeen revised accordingly in parallel to this document.
3- Overheat element installation improvements
(a) Heat conduction from attachment device of sensing element FIN 33HF and FIN 60HF
Improvements of overheat sensing element installation in pack bay (on elements FIN 33HF and FIN
60HF) by introduction of a Teflon spacer to avoid local thermal conductance.This modification is covered by aircraft MOD 28716 and embodiment rank is:
* For A319: MSN 1176 * For A320: MSN 1177 * For A321: MSN 1185
In this area it has also been noticed that the insulation blanket PN D3617023000400 was too short. This could lead to contact between overheat sensing elements FIN 33HF and FIN 60HF and bleed air ducting.
In order to prevent such contact, the length of this insulation has been increased. New blanket PN is D3617023000600. IPC 36-12-03 Fig 20 & 20A will be revised accordingly for the May 2000 revision of the IPC.
This modification is covered by aircraft MOD 28584 and embodiment rank is: * For A319: MSN 1022 * For A320: MSN 1003 * For A321: MSN 1021
These two items are covered by TFU 36.22.00.007 Overheat element installation in pack bay. These two modifications are introduced on in-service aircraft by SB A320-36-1039 released in April 2000.
(b) Heat radiation from air conditioning Flow Control Valve
Dedicated study of the ‘AIR BLEED’ maintenance status associated to a L ( R ) WING LOOP A ( B) fault message demonstrated that nearly 90 percent of the recorded occurrences are related to the loops ‘A’ (left and right). Review of the fault repartition within the fleet showed 75 percent of the occurrences on the aircraft before MSN 511.
MSN 511 corresponds to the introduction point of the rerouting of overheat sensing elements FIN 73HFand FIN 74HF above the ECS packs.
This rerouting has been defined in order to limit the impact of the Flow Control Valve (FCV) heat radiation during operation. As this heat radiation affected only the two loops ‘A’, it could result in an‘AIR BLEED’ maintenance status associated to a L ( R ) WING LOOP A fault message, or in a leak warning
when loop ‘B’ is already INOP.
The rerouting of the elements FIN 73HF and FIN 74HF is covered by the aircraft MOD 24001 for production aircraft and covered by Service Bulletin A320-36-1025 for in-service aircraft.Compliance of this service bulletin will change from ‘desirable’ to ‘recommended’ through an SB
revision in December 2000. Embodiment rank for MOD 24001 is:
* For A319: MSN 546 (All A319)* For A320: MSN 511
* For A321: MSN 513
B- EXPERIENCE WITH BLEED AIR LEAKS
(This paragraph is today limited to aircraft PRE MOD 49771 / PRE ABS1040 seals)
According to the experience of Airbus, bleed air leaks are mainly due to local leaks of a combo seal or flexible joint seal where the leak level increases the ambient temperature around the overheat sensing elements above its trigger threshold. An ageing review confirmed that, concerning bleed leaks, the bleed air ducting connections are mainly affected by flight cycles, rather than flight hours and by external ambient temperature when aircraft are operated on ground in high temperatures.
In order to improve the reporting of leak events from the field Wise article Engops-16477 (SIL 36-046) has been issued. Data collected in this way will be compiled and issued through update of Wise article Engops-16477 for the benefit of all operators.Fig 4 and Fig 5 summarize these data.
These two schematics are intended to assist operators by identifying the probability of leak location for right and left ‘AIR WING LEAK’ warnings.
Schematics given hereafter provide a preliminary estimation of what is the operator experience with leaks, which resulted in wing leak warnings.
Note: Inputs collected for the schematic hereafter concern only aircraft fitted with PRE ABS1040 seals (PRE MOD 32027)
C- EXPERIENCE WITH BLEED AIR LEAKS
The purpose of this paragraph is to provide operators with our experience regarding some preventive actions which, when applied, have led to a significant reduction of leak events.
(1) Leak check
Leaks are most of the time resulting from a degradation of a combo seal or flexible joint seals. Generally these seals have to be changed only on condition further to troubleshooting initiated by leak warnings. Due to the numerous man-hours generated by the trouble shooting and seals replacement, each leak event results in a delay if not a cancellation. In order to prevent delays and cancellations due to bleed leaks, it was decided to perform preventive leak checks at the opportunity of scheduled maintenance when access panels are opened.
The aim of these preventive leak checks is to clear the bleed air ducting from leaks which today are not detected by the overheat detection system because they are not large enough. This is because these leaks are due to leaking seals which, if they degrade further or if they are operated in hotter environment, could result in an overheat being detected, with consequential operational interruption.
In order to provide operators with an adequate preventive action planning it has been decided to have this planning covered by the MPD (Maintenance Planning Document).
For the post MOD 32027 aircraft the ABS1040 introduction will be closely monitored in addition to the in- service evaluation already launched. Preventive leak checks are maintained and the interval of these checks will be adjusted/extended based on the first results of leak checks performed on the first modified aircraft and experience collected from aircraft participating in the in-service evaluation.
As with the introduction of the ABS1040 seals some MPD tasks require to be revised, these revisions are listed hereafter and have been incorporated in the A320 MPD for the revision 26 of the MPD issued October 2002.
At 12000 FH interval on all aircraft Pre MOD 32027 / Pre SB 36-1043 (issue date planned April 2002):
- MPD task 361100-05-1 recommends to remove and replace the combo seals on the pack inlets including on the sleeves downstream and upstream of the FCV.
At 12000 FH interval on all aircraft:- MPD task 215200-07-1 recommends to remove and replace the combo-seals and peri-seals on the
pack ducting.
At 40 Months (12000 FH or 9000 FC) interval on all aircraft:- MPD task 361100-06-1 recommends to perform a leak check on the combo seals on the pylon duct sleeves (downstream precooler).
At 40 Months (12000 FH or 9000 FC) interval on pre MOD 28414 aircraft:
- MPD task 361100-07-1 recommends to perform a leak check on the flexible seals on the pylon towing ducting and on the wing to fuselage flexible ‘S-Duct’.
At 22000 FH interval on all aircraft:
- MPD task 361148-01-1 recommends to perform a leak check on the bleed air ducting in the wing area between bleed air precooler, wing anti ice valve and wing/fuselage connection.- MPD task 361200-03-1 recommends to perform a leak check on bleed air ducting in the fuselage area between the APU bleed check valve, air conditioning pack and wing/fuselage connection
At 22000 FH interval on A319:MPD task 361200-02-1 recommends to perform a leak check on the APU bleed air ducting (upstream the APU bleed check valve).
At 22000 FH interval on A320:
MPD task 361200-02-2 recommends to perform a leak check on the APU bleed air ducting (upstream the APU bleed check valve).
At 22000 FH interval on A321:MPD task 361200-02-3 recommends to perform a leak check on the APU bleed air ducting (upstream
the APU bleed check valve).
(2) Leak monitoring
In case of difficulties to identify the source of a leak (because this leak cannot be reproduced on ground) it is possible to monitor further the bleed air ducting for potential leaks by installation of temperature templates (Tool Part Number 08STHEB).
The purpose of these templates is to identify hot temperature areas. This is done by installation of these templates all along the ducting to be monitored for leaks. Identification of a hotter area is then managed by comparison of the temperatures recorded by the templates.
These temperature templates (Tool Part Number 08STHEB) can be wrapped around overheat sensingelements as per Fig 6. With this kind of installation elements will monitor the ambient air temperature compared to an installation directly on the ducting for which the duct skin temperature would be monitored.
Experience has shown that temperature templates (Tool Part Number 08STHEB) in the range between 70 and 110 °C (160 to 230 °F) would be the most appropriate to detect leaks in the range of 124 °C (255 °F).
(3) Aircraft Upgrade to ABS0737
Operators reported a significant reduction of bleed leak warnings as soon as all old standard seals are replaced by ABS0737 standard seals (Wise article Engops-16389 refers). Note that there is more benefit to replace all the combo seals by the new ABS0737 standard in one step for one complete section (wing, fuselage, ...), than in multiple steps. If done in one step, it is easier to limit the risk of incorrect ducting alignment.
Note: This recommendation is cancelled by the introduction of the ABS1040 PTFE seals.But it is also recommended to replace all previous silicon seals in one step (as per SB A320-36-1043) in order to gain complete benefit from the ABS1040 seal standard, and trace embodiment on aircraft with regard to the MPD changes introduced by this new seal standard.
(4) Overheat detection system check
It is recommended to follow the TSM for overheat detection system check. Also, if required, to carefully test the overheat detection loops with adequate tooling.
Details of overheat detection system check are given in Wise article EngOps-16312 (SIL 36-032) which has been introduced in TSM.36-22-00 page block 801.
(5) Aircraft operation on ground with high OAT:
(a) FCOM
Operators have reported that it was possible to significantly reduce the number of bleed air leak warning occurrences when the aircraft is parked during transit with a high outside air temperature by leaving the slats extended at position 1.This is covered by FCOM 3.03.24 Rev 30.
(b) TSM
Operators have reported that after repetitive occurrences of AIR BLEED maintenance status associated with L( R ) WING LOOP A(B) class 2 fault messages, it was not possible to dispatch the aircraft as no leak or overheat detection anomaly was found during trouble shooting and fault was still present.
In order to dispatch the aircraft when no bleed air leaks are found during troubleshooting with high OAT, the trouble shooting tasks which cover AIR BLEED maintenance status associated with L( R ) WING LOOP A(B) class 2 fault message have been updated in August 2000 with the following note:
The overheat detection threshold is between 117°C and 131°C (242°F and 268°F). An overheat detection by one loop can occur in these conditions:- High Outside Air Temperature (OAT)
- ECS supplied with hot bleed air.
In these conditions, the ambient air around the sensing elements can increase to the detection threshold but there is no air leak.If there is an air leak, there will be an AIR L ( R ) WING LEAK warning on the ECAM. If:
- The overheat detection was by one loop only with high OAT and ECS supplied with hot bleed air, and- The result of the BMC BITE test is TEST OK, no other maintenance action is necessary.
5. AIRBUS ACTION:
Hereafter is a table listing all the modifications listed in the present document:
For details, refer to paragraph ‘4. AIRBUS ACTION’.
6. MATERIAL:
Not applicable.
7. PROCUREMENT:
Purchase and repair orders relating to this document are to be submitted to:
Airbus Material Support Center
P.O. Box 63 02 62
D-22312 Hamburg Germany
Or to,
ANA spares center
21780 Filigree court
Ashburn
Virginia 20147-6205
Or to,
Airbus Spares center
C/O Lucas Aerospace PTE LTD
35/37 Loyang Way
1750 Singapore
Or to,
Airbus Office, Beijing Support Center Building Tianzhu Donglu 5
Tianzhu Airport industrial Zone
Shunyi Country
Beijing 101312
P.R. of China |
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