Advisory Circular (AC) No. 500-006 Issue 1

Aircraft Operations After Ground Cold Soak

File No. 5009-6-500 AC No. 500-006
RDIMS No. 528247-V3 Issue No. 01
Issuing Branch Aircraft Certification Effective Date 2004-12-01

1.0 Introduction

1.1 Purpose
1.2 Guidance Applicability
1.3 Description of Changes
1.4 Termination

2.0 References

2.1 Reference Documents
2.2 Cancelled Document

3.0 Background

4.0 Acceptable Techniques

4.1 Test Vehicle
4.2 Test Temperatures
4.3 Test Time
4.4 Instrumentation
4.5 Test Procedures
4.6 Test Witnessing
4.7 Test Results
4.8 Operating Experience

5.0 Aircraft Flight Manual

6.0Headquarters Contact

1.0 Introduction

1.1 Purpose

The purpose of this Advisory Circular (AC) is to provide guidance with respect to establishing and conducting a test program to demonstrate compliance with the requirements of Chapters 523 (as applies to Commuter Category Aeroplanes), 525, 527 and 529 of the Airworthiness Manual (AWM) concerning operation of the aircraft after prolonged exposure to cold temperatures on the ground or cold soak.

1.2 Guidance Applicability

This document is applicable to all Transport Canada personnel, delegates and industry.

1.3 Description of Changes

This document, formerly AMA No. 500C/5B, is reissued as an AC. The reference documents and the content have been reviewed and updated.

1.4 Termination

This document does not have a terminating action. It will however, be reviewed periodically for suitability of content.

2.0 References

2.1 Reference Documents

It is intended that the following reference materials be used in conjunction with this document:

(a) Chapter 523 of the Airworthiness Manual (AWM) - Normal, Utility, Aerobatic and Commuter Category Aeroplanes;
 
(b) Chapter 525 of the AWM - Transport Category Aeroplanes;
 
(c) Chapter 527 of the AWM - Normal Category Rotorcraft; and
 
(d) Chapter 529 of the AWM - Transport Category Rotorcraft.

2.2 Cancelled Document

As of the effective date of this document, AMA No. 500C/5B dated 2 March 1990 is cancelled.

3.0 Background

Past experience has indicated that cold soaked aircraft systems do not always function as predicted despite cold temperature testing conducted in laboratories on individual components.

The results of a cold soak affect the total aircraft as a system, not just the engine or individual components. The following difficulties could be encountered following a period of aircraft ground cold soak. This list is not exhaustive but is intended to be indicative of the difficulties, which have historically been identified and associated with aircraft ground cold soak:

(a) Hydraulic oil seals may become hardened at very cold temperature, resulting in leakage from landing gear and flight control servo actuators, thereby affecting mechanical reliability. Landing gear oleos may also leak for the same reason.
 
(b) Pneumatic lines may clog with frozen condensation, which could affect the operation of pneumatically operated systems.
 
(c) Fuel could start to congeal (slush) at very low temperatures and cause fuel filter blockage.
 
(d) Reduction gear oil seals could harden, causing the loss of oil and perhaps the loss of pressure.
 
(e) Metal shrinkage in metal components built to close tolerances, especially with dissimilar metals, could result in stiffness of controls, jamming of panel doors or other effects.
 
(f) Flight control take-off position indicator sensors may malfunction due to the effects of cold soak upon the components. There may also be effects on electrical trim motors resulting in slow trim rates or trim motor cut-out due to torque overload induced by stiffness of the controls.
 
(g) Lubricants may harden, resulting in stiff mechanical engine and flight controls.

Transport Canada has developed and applied a requirement to conduct testing after prolonged exposure to cold temperatures on the ground to ensure satisfactory cold temperature functioning of the aircraft. This requirement was originally approved and published as an additional airworthiness requirement in sections 525.1301-1, 527.1301-1 and 529.1301-1 of the AWM. This requirement was also applied for the type approval of SFAR 41C aeroplanes and is now incorporated in Chapter 523 and is applicable specifically to Commuter Category Aeroplanes.

To satisfactorily accomplish the objectives of the cold soak test, a systematic check of the operation of components and systems is required to show that the aircraft and its systems function properly and do not introduce hazards to safety.

The cold soak test should be of sufficient detail to give reasonable assurance that the aircraft systems will continue to function properly in service and that no potential effect on safety will result from the low temperature soak.

Failures or system anomalies that occur during the test should be fully documented and analyzed to determine the effect on aircraft design, and on maintenance and operating procedures.

4.0 Acceptable Techniques

4.1 Test Vehicle

An aircraft in production configuration, or equivalent, should be used for the test. Where there are deviations from the production configuration a review of the differences should be carried out to determine their impact on the test and the need for additional cold temperature testing. A document outlining the type design configuration deviations, as well as a rationale for their acceptability for the test, should be submitted to Transport Canada for review and acceptance prior to conducting the test. An aircraft conformity inspection and/or engineering inspection should be considered. The inspection(s) should be conducted prior to cold soak testing. The inspection(s) will help determine whether the aircraft design and configuration are appropriate for the intended testing. At the discretion of Transport Canada, the test article inspection(s) may be required. Where practicable, the configuration of the test aircraft should include worn or high life/life cycled components to determine the effect of cold temperatures. However it is not necessary that a large number of changes be made to the test aircraft and it is acceptable for new aircraft with limited flight time, to be used.

Where major modifications to the type design are subsequently implemented (e.g. engine change) a further review of the need to carry out additional testing, based on complexity of the changes, should be undertaken.

4.2 Test Temperatures

Sections 523.1301-1, 525.1301-1, 527.1301-1 and 529.1301-1 of the AWM require that satisfactory operation of the aircraft be substantiated after exposure, with its engine(s) shut down, to ground ambient temperatures equal to or less than -35ºC unless an alternative minimum ground ambient temperature has been proposed by the applicant and accepted by the Minister.

Past experience has shown that -35ºC is a realistic low temperature and is an achievable temperature for test purposes. This temperature has been selected as a practical temperature for a system interface test; however, satisfactory completion of the test is not to be interpreted as permitting alleviations from more severe component design criteria.

It is recognized that the ground ambient temperature may vary during the soak period and may be warmer than -35ºC at the start of the flight test part of the test and judgement must be used in evaluating the temperature profile against the intent of the requirement (e.g. if the cold soak is carried out overnight based on predicted colder temperatures the following day).

4.3 Test Time

The exposure time will vary depending on the type of aircraft and the environment prior to the start of the test. The test should reasonably simulate an overnight cold soak of the aircraft and the exposure time should be at least 10 hours, unless it can be shown that the systems and equipment (including interior) have stabilized at or below the required test temperature.

4.4 Instrumentation

Operation of the aircraft systems during the test will generally be monitored through the onboard system instruments and crewmember evaluation. Comprehensive instrumentation will not normally be required unless specific potential cold temperature related problems have been previously identified.

Sufficient temperature monitoring devices should be used to reasonably identify the condition of the aircraft in particular the stabilization temperature. Where exposure to the cold temperature is considered to be sufficiently long, local recorded temperatures may be sufficient (e.g. control tower).

4.5 Test Procedures

The applicant should submit a proposed test procedure to Transport Canada for review and acceptance detailing the checks to be carried out and the action to be taken in preparation for the test. The test procedure should be submitted to Transport Canada in a time frame sufficiently in advance of the test that is acceptable to Transport Canada.

During the test, published or proposed procedures for operation in a low temperature environment should be used. Where the test procedure makes reference to published or other documents, those documents should be provided to Transport Canada. All deviations from the procedures and anomalies that occur during the test should be recorded.

The test procedure shall specify any inspections or procedures, in addition to those presented in an existing Aircraft Flight Manual (AFM)/Aircraft Maintenance Manual (AMM), that are required to prove correct functioning of a system or component. Test procedures may also be specified in order to establish additional instructions not yet in the AFM/AMM.

The detailed test procedures may vary from aircraft to aircraft, however, the following is indicative of the procedure that would be expected and should be included in the detailed procedures document.

(a) Preparation prior to cold soak
 
(i) Prepare the aircraft for extended cold soak in accordance with the published or proposed procedures. Any additional preparation tasks accomplished (e.g. application of covers/protections for engines and air intakes, removal of batteries) would be added to the cold soak procedures in the AFM/AMM.
 
(ii) Conduct an exterior inspection of the aircraft in accordance with the approved AFM and/or AMM, and any other acceptable procedures.
 
(iii) Record (and adjust if necessary) system fluid levels and pressures (e.g. Hydraulic system accumulator pressures, oleo extension, tire pressures) and adjust to normal operating range where necessary. Measure cable tension in primary flight control cables.
 
(iv) Refuelling of the aircraft to the critical level should be carried out prior to cold soak. If refuelling is carried out at the end of the cold soak period, (e.g. during paragraph 4.5(c) pre-flight preparations), this would become the cold soak procedure in the AFM/AMM. The method to be followed must be identified in the test procedure.
 
(b) Preparation during cold soak
During the cold soak, record ambient temperature and, if an on board temperature monitoring system is installed, aircraft temperature at intervals sufficient to substantiate temperature profile during test.
 
(c) Preparation after cold soak
After satisfactory aircraft temperature has been established, the test should include but not necessarily be limited to:
 
(i) Conduct an exterior inspection of the aircraft in accordance with the approved AFM and/or AMM.
 
(ii) Record system fluid levels and pressures. Measure cable tensions in the primary flight controls. Note any changes from pre-test conditions.
 
(iii) Restore aircraft to operational mode in accordance with the published or proposed procedures.
 
(iv) Check all external doors (including emergency exits) for correct functioning.
 
(v) Conduct normal AFM pre-flight procedures and record engine start parameters, times, etc. The battery current and voltage output during engine start should be recorded, such that it is possible to determine whether the removal of the battery from the aircraft prior to cold soak is to become a limitation. The time from APU start up/engine start to the ready for take-off condition is to be recorded and included in the test report. An instruction to make such a record should be included in the test procedure. The test must be fully representative of the in-service procedure.
 
(vi) Function systems through operating range and check auxiliary systems.
 
(vii) Conduct take-off and carry out AFM checks procedures.
 
(viii) Carry out system checks and exercise all alternate and back-up functions/modes. The alternate and back-up function/modes do not have to be exercised if the test is impractical given the conditions (e.g. requires disabling a sensor). If all functions/modes are not exercised, justification and supporting data substantiating why the functions/modes could not be exercised and what significance the failure to exercise them might have on the interpretation of the test results should be submitted to Transport Canada.
 
(ix) Conduct pre-landing checks and appropriate shut-down procedures.
 
(x) Conduct an exterior inspection of the aircraft and record system fluid levels and pressures.

4.6 Test Witnessing

The cold soak test is carried out under natural environmental conditions that may complicate the test since the desired conditions may be difficult to achieve and maintain. The aircraft systems often, under extreme conditions, display degraded or unusual performance. It may be necessary due to challenging or changing test conditions and/or aircraft system performance to make field decisions regarding the test that requires the concurrence of the responsible Transport Canada specialist. Additionally, it is expected that it would be difficult to duplicate the actual test conditions in the event a test needed to be repeated or the interpretation of the test results are questioned. The resolution of any issues arising from the testing would be facilitated by the presence of a Transport Canada specialist. Therefore, where practical, the cold soak test should be witnessed by Transport Canada or a delegated representative.

4.7 Test Results

A report of the cold soak test should be submitted to Transport Canada. The report should contain sufficient information to demonstrate compliance with the appropriate sections (523.1301-1, 525.1301-1, 527.1301-1 or 529.1301-1) of the AWM. Data, such as times and pressure, should be included where the cold soak test plan requires various parameters to be measured and/or recorded during the test.

The requirement is intended to test the effects of cold soak on the total aircraft system. Compliance with the requirement as it pertains to the total aircraft must be evident.

All deviations from procedures and anomalies identified during the test should be documented. Anomalies should be analyzed to determine the cause and identify necessary remedial action. In reviewing the anomalies, consideration should be given to their effect on such aspects as aircraft design, AFM limitations and procedures, and maintenance procedures.

Where practical, review of anomalies, identified during the test, should take into consideration adverse limits of permissible wear or life cycles.

4.8 Operating Experience

Where the aircraft has sufficient experience of satisfactory operation in regions of low ground ambient temperatures, credit may be given toward testing for compliance to the requirements subject to adequate substantiation. Such substantiation should include reasonable evidence of operation at the low ground temperature including data related to periods of ground cold soak.

A historical record of known cold temperature related problems should also be provided and the effects of the in-service difficulties on aircraft design, AFM limitations and procedures and maintenance procedures should be presented.

5.0 Aircraft Flight Manual

Where cold soak test temperature of -35ºC has not been achieved and/or compliance with the requirement has not been substantiated, limitations reflecting the specific ground ambient temperature determined on the basis of the actual cold soak temperature achieved will be required in the AFM. In such cases, a limitation similar to one of the following will be required at the discretion of Transport Canada, as appropriate to the test results achieved:

(a) A statement in the Limitations Section that operation is prohibited if the aircraft, with its engine(s) shut down is exposed to a ground ambient temperature below a stated temperature, which is the actual minimum cold soak temperature achieved (e.g. engine start below an ambient temperature of -20oC is prohibited). This limitation is used where a prolonged cold soak has been demonstrated to an average temperature higher than -35ºC and no demonstration has been conducted to identify aircraft performance following a brief exposure (less than 10 hours) to a temperature lower than -35ºC.
 
(b) A statement in the Limitations Section that operation of the aircraft is prohibited if the aircraft, with its engine(s) shut down, has been exposed to a ground ambient temperature below a stated temperature without application of an accepted procedure, (e.g. "Engine start after exposure to temperatures below x ºC is prohibited without application of the following procedure./ the procedure specified in.). This limitation is used where cold soak has been demonstrated to -35ºC or lower with the use of an acceptable procedure that may appear in the AFM or AFM Supplement depending on the procedure complexity. The "x ºC" is the temperature at which the aircraft was demonstrated without the use of the accepted procedures. The procedure itself may vary considerably (e.g. use of external power, to the requirement to use specified enclosures and heaters).
 
(c) A statement in the Limitations Section that operation is prohibited if the aircraft, with its engine(s) shut down, has been exposed to a ground ambient temperature below a stated temperature in excess of a specified period of time, without application of an accepted procedure (e.g. Engine start after exposure to temperatures of x ºC for more than y hours is prohibited without application of the following procedure./ the procedure specified in.), where "y" is the actual period of cold soak time achieved at the "x" temperature.

This limitation is used where cold soak (10 hours) has been demonstrated successfully to a temperature higher than -35ºC and also demonstrated to a temperature of -35ºC or lower for a limited period of time (less than 10 hours) without the use of specific procedures and it has been demonstrated that if this time period is exceeded then the use of acceptable procedure will be required in order to permit safe flight. This procedure may appear in the AFM or AFM Supplement depending on the procedure complexity, and the procedure itself may vary considerably (e.g. use of external power, to the requirement to use enclosures and heaters).

6.0 Headquarters Contact

For more information please contact:

Policy Standards Coordinator (AARDH/P)
Phone: (613) 990-3923
Facsimile: (613) 996-9178
E-mail: AARDH-P@tc.gc.ca

Original signed by Maher Khouzam

Maher Khouzam
Chief, Regulatory Standards
Aircraft Certification Branch

Date modified: