|What the JAR's Say about Auto-Land Fail-operational Modes|
Subpart NotesDate: August 1, 1996
An aeroplane with a basic airworthiness approval for IFR operations is eligible to perform ILS precision approaches down to a decision height of 60 m (200 ft), assuming that the necessary ILS receiver(s) and instruments and their installation have been approved. The supplementary airworthiness criteria for aeroplanes to perform ILS precision approaches down to a decision height below 60 m (200 ft) and down to 30 m (100 ft) is contained in Subpart 2.
The purpose of this Subpart 3 is to specify the supplementary airworthiness criteria for aeroplanes to perform ILS precision approaches with decision heights below 30 m (100 ft) or with no decision height.
This material may not be appropriate to precision approach aids other than ILS.
It should be noted that when other guidance information is used to supplement the ILS (e.g. inertial navigation systems) some reduction may be acceptable in the standard of ILS ground station indicated below.
The criteria are divided, where necessary, into those applicable to the following types of operation:
(1) Decision heights below 30 m (100 ft) but not less than 15 m (50 ft);
(2) Decision heights below 15 m (50 ft);
(3) No decision height.
Background information regarding the characteristics of these types of operation is presented in more detail in the paragraphs B, C and D which follow.
B Decision Height below 30 m (100 ft) but not less than 15 m (50 ft)
The Runway Visual Range (RVR) required by a pilot to make the decision to land from a decision height below 30 m (100 ft) is less than he would need at 30 m (100 ft). Furthermore, the time from the decision height to the start of the flare manoeuvre will be less.
Consequently, in order to achieve the desired success rate and to preserve the safety level, it has been considered necessary that the aeroplane be fitted with an automatic landing system. Use of such a system also ensures that the aeroplane is within the obstacle-free zone specified in ICAO Annex 14 during approach and any go-around so that there is no need to take obstacle clearance into account in determining the decision height. This is chosen to give an acceptably low probability of touching the ground during go-around.
The minimum RVR provides visibility at and below the decision height so that, if either the automatic landing system or the ILS fails when the aeroplane is below the decision height, the pilot can carry out a manual landing with an acceptable safety level.
The ground guidance system is a Category III ILS or a Category II ILS which complies with the Category III standards of ICAO Annex 10 in respect of all significant performance parameters, down to ILS Point D, 900 m (3,000 ft) from threshold.
C Decision Height below 15 m (50 ft)
Aeroplanes which have a fail-operational landing system, can be certificated for operation with a decision height below 15 m (50 ft).
In this type of operation, the RVR needs not only to be sufficient for the pilot to make the decision at the decision height, but also to be sufficient to enable the pilot to control the aeroplane during the ground roll. The main purpose of the decision height is so that he can assess the adequacy of the visibility before touchdown and prepare to take over visual manual control. It is desirable that the decision height be late in the flare after the major pitch changes have taken place, and that an automatic go-around system be fitted. There exists an unknown probability that, although the visibility is reported to be adequate, denser patches of fog may lie on the runway, and it is thought prudent to add a margin to the bare minimum required to control the ground roll. The RVR limit is usually set in the range 200 m to 150 m but may be lower if the aeroplane has either an automatic ground roll control or a head-up ground roll guidance display which could be used if the visibility falls below the minimum necessary.
The ground guidance system (ILS) is as described in paragraph B, and, additionally, complies with a continuity of service objective (failure survival capability) of 2 x 10-6. It is assumed that the pilot is promptly notified by ATC of a failure or degradation of the required ground equipment (e.g. loss of stand-by ILS transmitter).
D No Decision Height
An aeroplane with a fail-operational landing system with automatic ground roll control (or ground roll guidance) may be certificated for operation without a decision height (operations when the pilot is not required to make a decision described in the definition of Decision Height). If the ground roll system is fail-operational, no RVR limit is necessary. If the ground roll system is not fail-operational, then an RVR limit is applied such that the pilot can control the aeroplane safely on the ground in all likely conditions.
In these visibility conditions, the pilot is likely to brake hard during the ground roll and therefore an anti-skid braking system is considered to be essential. Distance and ground speed indications and automatic braking would obviously be useful, but are not considered to be essential and are not required.
The ground guidance system (ILS) complies with the Standards of ICAO Annex 10 for Category III ILS, and, additionally, complies with an integrity objective of 10-9 and a continuity of service objective of 2 x 10-6.
As used herein, the terms listed below have the following meaning:
The term 'landing system' used here refers only to the airborne system. It includes the equipment listed in JAR-AWO 321 and also all related sensors, instruments and power supplies.
Automatic Landing System
The airborne equipment which provides automatic control of the aeroplane during the approach and landing.
Fail-passive Automatic Landing System
An automatic landing system is fail-passive if, in the event of a failure, there is no significant out-of-trim condition or deviation of flight path or attitude but the landing is not completed automatically.
NOTE: For a fail-passive automatic landing system the pilot assumes control of the aircraft after a failure.
The following are typical arrangements:
(1) A monitored automatic pilot in which automatic monitors will provide the necessary failure detection and protection.
(2) Two automatic pilots with automatic comparison to provide the necessary failure detection and protection.
Fail-operational Hybrid Landing System
A system which consists of a primary fail-passive automatic landing system and a secondary independent guidance system enabling the pilot to complete a landing manually after failure of the primary system.
NOTE: A typical secondary independent guidance system consists of a monitored head-up display providing guidance which normally takes the form of command information, but it may alternatively be situation (or deviation) information.
The alert height is a specified radio height, based on the characteristics of the aeroplane and its fail-operational landing system. In operational use, if a failure occurred above the alert height in one of the required redundant operational systems in the aeroplane (including, where appropriate, ground roll guidance and the reversionary mode in a hybrid system), the approach would be discontinued and a go-around executed unless reversion to a higher decision height is possible. If a failure in one of the required redundant operational systems occurred below the alert height, it would be ignored and the approach continued.
Decision height is the wheel height above the runway elevation by which a go-around must be initiated unless adequate visual reference has been established and the aircraft position and approach path have been assessed as satisfactory to continue the approach and landing in safety.
Where it is used in this document it means the minimum decision height determined in the airworthiness certification.
Minimum Approach Break-off Height
The Minimum Approach Break-off Height (MABH) is the lowest height of the wheels above the runway at which it is demonstrated that a go-around can be carried out without external visual references according to the standard procedure, with an acceptable level of safety. (See JAR-AWO 313 and 314.)
A go-around is the transition from an approach to a stabilised climb.