CS 25.745 Nose-wheel steering

In a powered nose-wheel steering system the normal supply for steering should continue without interruption in the event of failure of any one power-unit. With the remaining power-units operating at ground idling condition, the power supply should be adequate – a. To complete an accelerate-stop manoeuvre following a power-unit failure which occurs during take-off, and b. To complete a landing manoeuvre following a power-unit failure which occurs during take-off or at any later stage of flight.

1 No failure or disconnection need be assumed in respect of parts of proven integrity e.g. a simple jack or manual selector valve, but slow leakage from pipe joints and fracture of pipes should be considered as probable failures. 2 In assessing where the inadvertent application of steering torque as a result of a single failure would lead to danger, allowance may be made for the pilot’s instinctive reaction to the effects of the fault. However, dependent on the urgency and rapidity of warning of the failure given to the pilot, allowance should be made for a reaction time before it is assumed that the pilot takes any corrective action.

CS 25.745(d) provides for the two following options:

1.A ‘no damage’ situation exists, because damage is precluded.

2.Damage can occur, but indication to the flight crew is provided.

(a)General consideration to CS 25.745(d)(1) and (2) Some damage may occur during ground manoeuvring activities that can be considered acceptable and judged to be normal wear and tear. It is not intended that such damage needs necessarily to be precluded or that it should initiate a flight crew alert.

(b)To comply with CS 25.745(d)(1) the following applies: The aeroplane may be designed in such a way that under all ground manoeuvring operations by any towing means, no damage affecting the steering system can occur. Examples are: The steering system is designed sufficiently strong to resist any applied towing input. The steering system is designed to allow 360 degrees rotation. The steering system is disconnected either automatically or by operational procedure. The steering system is protected by shear sections installed on the nose landing gear.

(c)To comply with CS 25.745(d)(2), the following applies: When protection is afforded by the flight crew alerting system, the damage detection means should be independent of the availability of aeroplane power supplies and should be active during ground manoeuvring operations effected by means independent of the aeroplane. If damage may have occurred, a latched signal should be provided to the flight crew alerting system.

(d)Alternative Acceptable Means of Compliance to CS 25.745(d)(1) and (2): In the case where the aeroplane design does not comply with CS 25.745(d)(1) and (d)(2), the following apply:

(1)The Aeroplane Flight Manual, in the Section Limitations, should include a statement that ‘Towbarless towing is prohibited’, or

(2)The Aeroplane Flight Manual, in the Section Limitations, should include a statement that: ‘Towbarless towing is prohibited unless the towbarless towing operations are performed in compliance with the appropriate operational regulation using towbarless towing vehicles that are designed and operated to preclude damage to the aeroplane nose wheel steering system, or which provide a reliable and unmistakable warning when damage to the steering system has occurred. Towbarless towing vehicles that are specifically accepted for this type of aeroplane are listed in the [appropriate maintenance documentation] provided by the aeroplane manufacturer.’ ‘Appropriate maintenance documentation’ means Instructions for Continued Airworthiness as described in Appendix H, paragraph H25.3(a)(4) of CS-25.

(3)The acceptance by the aeroplane manufacturer of the applicable towbarless towing vehicles and its reliability of the oversteer protection and/or indication system as referred to in subparagraph ((d)(2)) above should be based on the following:

(i)The aeroplane Nose Wheel Steering Failure Analysis should include the effects of possible damage caused by towbarless towing operations.

(ii)If the Nose Wheel Steering Failure Analysis shows that damage to the steering system by the use of towbarless towing may result in a Failure Condition that can be classified as Hazardous or Catastrophic (refer to CS 25.1309), the acceptance of a towing vehicle oversteer protection and/or indication system should be based on an aeroplane safety analysis, encompassing the reliability of that vehicle system in order to meet the aeroplane safety objectives.

(iii)If the Nose Wheel Steering Failure Analysis shows that damage to the steering system by the use of towbarless towing may result in a Failure Condition that can be classified as Major or less severe, the aeroplane manufacturer can accept the design of the towing vehicle oversteer indication and/or protection system based on a ‘Declaration of Compliance’, issued by the towbarless towing vehicle manufacturer. This declaration will state that the vehicle design complies with the applicable standards (SAE ARPs, Aeroplane Towing Assessment Criteria Document) and that it is designed and built under ISO 9001 quality standards or equivalent. Such a declaration must be made regarding all Towbarless Towing Vehicles to be used for ground manoeuvring of CS-25 certificated aeroplanes.

[Amdt 25/13]