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RTLU AIRBUS

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RUDDER TRAVEL LIMITATION UNIT MECHANICAL STOP FAILURE
Reference: 27.23.00.007
Issue date: 02-APR-2021
Last check date: 02-APR-2021
A/C type/serie: A318, A319, A320, A321
ATA: 27-23
Engine manufacturer:
Supplier: ARTUS SAS
Purpose / Reason for revision: Please refer to TFU_27.23.00.007_Summary.pptx document in attachment
Engineering Support
Status: Closed
Status: Closed
Applicability: ALL Single Aisle Aircraft
Reference / Documentation
 TFU 27.23.51.005: Rudder Travel Limitation Unit mechanical stop.
 ISI 27.20.00001 (old SIL 27-107): inhibition of the Rudder Travel Limitation Unit "back-up" function during maintenance.
 AMM 22-61-00 PB001: ARTIFICIAL FEEL AND RUDDER TRAVEL LIMITING COMPUTATION.
 AMM 27-50-00-866-008-A01 - Extension of the Flaps/Slats on the Ground.
 FCOM SOP After landing / Securing the Aircraft / Cockpit Preparation.
 TFU 36.22.00.029 High temperature in pack bay area at X-bleed valve location.
Description
Some operators have reported high number of RTLU found with mechanical stops damaged. The RTLU affected by mechanical
stops damaged have various S/N from old to recent units.
Most of the reports came from operators operating in hot weather condition.
Fig 1. Typical finding in shop: mechanical stops with fork destroyed
Consequences
Broken mechanical stops may cause jamming of RTLU control screws due to erratic rotation of the bushes.
Incorrect behaviour of the RTLU is then detected by both FACs and the failure of both RTLU channels is triggered.
Consequence is the warning AUTO FLT RUD TRV LIM SYS which is a NO GO item associated to the failure message AFS:
FAC1/RTL ACTR 4CC and AFS: FAC2/RTL ACTR 4CC.
In most of the cases, the fault could not be reset on ground and the replacement of the RTLU was necessary to fix the fault.
Affected operators reported many Operational Interruptions (OI), in particular when the failure popped up in pre-flight or
flight phase 02 and a low reliability of the RTLU over their fleet, with a MTBUR below the GMTBUR while fleet MTBUR is above.
Investigation Status
A too frequent activation of the "return to low speed" function is understood to be the cause.
The Rudder Travel Limit Unit (RTLU) is an electrical actuator that aims at limiting the maximal rudder deflection to avoid
exceeding max allowable loads on the rudder. The highest the airspeed, the lowest the allowed rudder travel.
The FACs computers are responsible to derive the Rudder Travel Limiting (RTL) function of the airspeed and also to control
and monitor the RTLU proper operation: opening/closure or the functional stops as expected.
Here below is given as example a simplified representation of the RTL control law for A320.
Fig 2. Simplified Rudder Travel Limit law for A320
When the below conditions are met simultaneously:
1. The FACs are no longer able to ensure basic RTL function - meaning not able to compute RTL control law or not able
to nominally control the RTLU –
and
2. The slats are extended (beyond 17°),
the Emergency "return to low speed" mode of the RTLU is activated.
Purpose of the "return to low speed" function is to allow recovering full rudder authority at landing and ensure optimal lateral
control at low speed (for ex landing with X-wind, engine failure…).
In this back-up mode, the RTLU motors are fed by 26VAC power supply in “open loop” mode, leading to open the RTLU
functional stops until the mechanical stops.
The "return to low speed" logic is illustrated on AMM figure 22-61-00-17900-00.
Investigations have highlighted the emergency function could “unduly” activate on ground when:
The TLU is supplied with 26VAC (FAC C/B closed)
and
1. Both FACs P/B are off and the slats are extended
or
2. ADR data are not available to FACs (ADIRU Off or not aligned) and the slats are extended
These conditions can be met:
- During maintenance operations with slats extended.
For information, AMM procedures for slats/flaps extension on ground were modified in 1994 to request opening of
FACs C/B 14CC1 and 14CC2 before slat extension (job set up). Please refer to ISI 27.20.00001 (old SIL ref 27-107)
that provides more information and appropriate recommendations.
-
During ground operations if slats are kept extended after landing.
Typically, in hot weather conditions, some operators keep the slats out after landing, as per FCOM recommendations,
in order to cool down the wing area and avoid spurious AIR WING LEAK ECAM alert, which is a NO GO alert. See
TFU 36.222.00.029 for more information.
Then, when switching off all three ADIRUs (as per “Securing the aircraft” FCOM procedure) or aligning all three
ADIRUs (Complete alignment during “Cockpit preparation” FCOM procedure) the FACs will no longer be able to
compute the RTL function and since the slats are extended, the “return to low speed” function will activate.
Under this last scenario, quasi daily activation of the function is possible when aircraft is operated in hot weather
conditions. The mechanical stops being seized to sustain only occasional activation of the "return to low speed"
function, this could result in damaged or broken mechanical stops.
Mitigation / Interim Plan
To avoid damages on the RTLU mechanical stops, FCOM was improved to remind to retract the slats before switching off all
three ADIRUs simultaneously.
This change has been incorporated in FCOM revision Sept 2013.
Maintenance Information
N/A
Repercussions on A/C Operations
N/A
Permanent Solution
FAC C software CAA09 and FAC B software B625 will introduce improve logic to inhibit undue activations of the “return to low
speed” on ground.
FAC C software CAA09 is introduced by MOD 162676 and SB 22-1629 (CEO) and SB 22-1630 (NEO) that were dispatched to the
operators on 23 MAR 18.
FAC B software B625 is introduced by MOD 163264, was certified in AUG 2018.
The modification is introduced on In-Service Aircraft by SB 22-1644, which was dispatched to the operators on 08
MARS 2019.
[Info ATA 36 - extract of TFU 36.222.00.029]: A new insulation muff PN36171234 was introduced in service mid of
2016. This new muff proved efficient with no WING LEAK ECAM alert.
Survey for the Engineering Support section
Retrofits Monitoring
Status: Closed
Retrofit Information Letter: YES
SB A320-22-1629 & SB A320-22-1630 – FAC CAA09
AIRBUS Monitored Retrofit has ended.
Standard commercial conditions from SB/VSB now apply.
Please contact AIRBUS Warranty Manager or AIRBUS Customer Support Director for further support.
For a complete list of monitored retrofits applicable to your fleet, please contact
monitored.retrofit@airbus.com
SB A320-22-1644 – FAC B625
AIRBUS Monitored Retrofit has ended.
Standard commercial conditions from SB/VSB now apply.
Please contact AIRBUS Warranty Manager or AIRBUS Customer Support Director for further support.
For a complete list of monitored retrofits applicable to your fleet, please contact
monitored.retrofit@airbus.com
Survey for the Retrofits Monitoring section
Annex
General Information
Potential impact: Maintenance, Operational Reliability
Key information: Environment - Hot Weather, Linked Items - FAIR
Solution benefit:
First issue date: 08-JAN-2015
Technical parameters
ATA: 27-23
A/C type/serie: A318, A319, A320, A321
Engine:
Engine manufacturer:
Fault code/ECAM
warning:
FIN: 4CC
Part Number: DV8456701-5
Supplier: ARTUS SAS
Attachments
General:
- TFU_27.23.00.007_Summary.pptx
Retrofits Monitoring:
- RIL SA22M18000648 Appendix.xls
- RIL SA22M18000648 R00.pdf
- RIL SA22M18002937 R00 appendix.xls
- RIL SA22M18002937 R00.pdf
Issue date: 02-APR-2021
Last check date: 02-APR-2021
Links
Other articles (ISI/TFU):
- 27.20.00001, 27.23.51.005
FAIR-ISP:
- 14.0060
© Airbus SAS, 2021. All rights reserved. Confidential and proprietary document.The technical information provided in this article is for convenience and information purposes only. It
shall in no case replace the official Airbus technical or Flight Operations data which shall remain the sole basis for aircraft maintenance and operation. These recommendations and
information do not constitute a contractual commitment, warranty or guarantee whatsoever. They shall not supersede the Airbus technical nor operational documentation; should
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