Radio Altimeter operation on aircraft - DOC

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                       International Civil Aviation Organization

                       WORKING PAPER



                                     Paris, France 17 – 21 March 2011

Agenda Item 6: Any other business

                                  Aircraft Operation of Radio Altimeters

                                    (Presented by John Taylor, Canada)

              This paper provides operational information on the use of Radio Altimeters
              onboard aircraft that form a critical component of aircraft certification that is
              necessary to support normal flight operation, including precision approach and
              automated landing.
              The meeting is requested to note the information and take it into account as
              necessary to ensure protection of spectrum in the band 4 200 – 4 400 MHz.


1.1              Radio altimeters onboard aircraft provide a measurement of altitude or height of the
aircraft above ground or terrain. From the perspective of aircraft certification, inclusion of dual radio
altimeters in the system configuration and flight operations concept is one of high importance. Radio
altimeters form a critical element in the aircraft’s Flight Management Guidance Computer
(FMGC) and Flight Control systems.

Modern commercial aircraft are highly automated, and use various levels of integrated computer
architecture to manage many onboard systems including primary flight control systems, system messages,
navigation, auto-flight including autothrust and autopilot, and engine controls. The current generation of
airline aircraft all have some form of computerized primary flight controls either for stability/flyability,
flight envelope protection or both. These systems are integrated to various degrees depending on the
manufacturer to provide levels of system resulting in reduced pilot workload and increased system
(5 pages)
ACP-WGF24/WP-09                                      -2-

capability. It is difficult to isolate the effect of one system on another in modern transport category
aircraft because of this integration; however valid radio altimetery information is critical to some or all of
these systems.

1.2                System faults and status are displayed by EICAS/ECAM type monitoring systems that
are the primary means of displaying airplane system information to the flight crew. These systems
consolidate engine and avionic subsystem indications to provide a centrally located crew alerting
function. Depending on the system architecture they display System Alerts (Warning, Caution, and
Advisory), Communication Alerts, and other essential status messages. While on the ground or in-flight
these automated systems acquire sensor data from a vast array of parameters that are essential to normal
operation of an aircraft, including but not limited to radio altimeter information. The status of alerts and
their impact on the operation of the aircraft follows a structured hierarchy, from critical, decreasing lower
in the fault tree to non-critical.

1.3              While on the ground there are system fault alerts that impact the certification, and
ultimately the flight dispatch certification of the aircraft. These system faults are identified in the
aircraft’s Master Minimum Equipment List (MMEL) where mandatory adherence by regulation applies to
the systems operating on the aircraft. Systems listed in the MMEL, or operators specific MEL, may be
dispatched inoperative, or degraded, under specific conditons where system redundancy permits safe
operation with the remaining system. In most cases dispatch with both systems inoperative is a “no
dispatch” item. As a multi-use sensor the loss of both Radio Altimeter systems is normally a no-go or no
dispatch item.


2.1             Considering a typical commercial aircraft of today, and the high level of automated
systems and sensors that are used to manage the aircraft’s integrated avionics and essential components it
is important to understand how specific components are used and how they impact the hierarchy of
system degradation during flight, degradation that can cause significant implications to the safe operation
of the aircraft while also increasing the crew workload as they manage amended flight operation

2.2             Radio altimeters onboard an aircraft do not operate as a sole independent avionics
component, moreover they are integrated into the complete aircraft flight management computer systems
and are also a critical component of the Ground Proximity Warning System (GPWS) or Terrain
Avoidance Warning System (TAWS), modern aircraft flight control architecture, and EICAS/ ECAM
systems. They are a critical component during low visibility operations and in the precision approach and
landing phase of flight, where the decision height of the aircraft to the runway surface must be accurately

2.3               The integration of radio altimeters as part of overall aircraft computer systems means that
reliable operation is mandatory not only by regulation, but also by design of the aircraft flight systems
architecture, ie; the computer systems software that controls the sensor data input necessary for normal
stabilised flight operation.

2.4              Typically a Master Minimum Equipment List (MMEL) is developed by a CAA, with
participation by the aviation industry, it’s purpose is to improve aircraft utilization and thereby provide a
more convenient and safe air transportation system for the public. The MMEL must not deviate from the
Aircraft Flight Manual Limitations, Emergency Procedures, Certification Requirements, or with
Airworthiness Directives. It is important to remember that aircraft can only operate with inoperative
                                                    -3-                             ACP-WGF24/WP-09

equipment related to the airworthiness and the operating regulations of the aircraft in accordance with the
conditions and limitations perscribed in the MMEL. The requirements for specific communications,
navigation, radar and collision avoidance systems is stipulated in ICAO Annex 6, and also in the ICAO
Manual of All Weather Operations are listed in the MMEL.

3.0             ANALYSIS

                3.1     Considering the integration of radio altimeters in the aircraft flight management
                        computer, and flight control systems, as a critical component, it is worthwhile to
                        provide a description of a flight profile under normal circumstances, and also
                        under abnormal circumstances.

                3.2a      For example, under normal circumstances if we analyse a landing phase flight
                profile from 10nm to the runway threshold, the avionic system components
                predominantly in use are the ILS receivers, DME, ADF, Marker, GPS, Radio Altimeter,
                Air Data Computers providing baro-altitude and airspeed, Inertial reference systems etc,
                moreover the flight management and flight control computers continuously monitor
                sensor data input and correlates this data to ensure it is within specific parameter limits,
                particularly that the radio altimeter height reading between the two sensors is correlated
                to be within tolerance. Auto-throttle is engaged, a stabilised approach with controlled
                descent rate and speed is maintained. At a pre-established decision height, the Glidepath
                vertical information sensor data is phased out of the equation by the flight management
                computer and the vertical height above the runway surface is provided by the radio
                altimeter with aural annunciation in feet to initiate flare of the aircraft to touchdown. This
                flight profile can be achieved in normal or low visibility conditions.

                3.2b     Under abnormal circumstances if we analyse the same landing phase profile from
                10 nm from the runway threshold, with the same avionics system components in use, the
                aircraft is established on a stabilised approach with auto throttle engaged, controlled
                descent rate and airspeed are maintained. At approximately the 200 foot decision height
                the flight management computer correlates radio altimeter data between the two sensors
                that due to errors exceeds the allowable tolerance. Depending on the aircraft type the
                following is what will immediately happen;

                a)      Autoland warning lights will illuminate.

                b)      The auto pilot may disengage.

                c)      The crew may have to abandon the approach.

                3.2c Under another abnormal circumstance if we analyse the same landing phase profile
                from 10 nm from the runway threshold, with the same avionics system components in
                use, the aircraft is established on a stabilised approach with auto throttle engaged,
                controlled descent rate and airspeed are maintained. At some point on the approach both
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                 Radio Altimeters lose valid information. Depending on the aircraft type the following is
                 what will immediately happen;

                 a)       Both autopilots will be lost. The crew will have to hand fly the approach.

                 b)       The flight control laws will revert to a direct law configuration. Stability and
                          flight envelope protections will cease.

                 c)       The approach capability will degrade to a Category 1 only. In low visibility
                          operations, this scenario would result in a go around missed approach situation

                 d)       The onboard monitoring system will provide system fault messages.

                 e)       Overall, this scenario compromises the safety of the aircraft by increasing the
                          crew workload significantly and degrading the approach capability.

4         This scenario can certainly place unnecessary demands on the flight crew and compromise safety
          of the aircraft. In a situation of this nature, if one were to consider a busy international airport
          where multiple aircraft are stacked at holding altitudes which creates increased airspace density,
          the workload situation for air traffic control also increases dramatically stemming from the fact
          that an aircraft has now initiated a go around from an arrival runway that is configured in the
          pattern flow for landing aircraft. In theory, the go around aircraft now has to be separated from
          other traffic.

4.1              It can be understood that radio altimeters support a very critical function in all phases of
flight, whether it is enroute (because of the interaction with GPWS and TAWS) or precision approach and
landing. The potential for disruption of normal radio altimeter operation stemming from the emissions of
non-aeronautical and non-safety service systems operating in the 4 200 – 4 400 MHz band would clearly
be unacceptable.


5.                CONCLUSION

5.1              The operational scenario described here is typical of aircraft operations that would occur
in any part of the World. Considering the desire of commercial broadband interests to gain access to the
frequency band 4 200 – 4 400 MHz, the potential for disruption to the current use of the band by radio
altimeters would be high.

5.2                The use of the frequency band 4 200 – 4 400 MHz by radio altimeters is in accordance
      with the Radio Regulations where the special provisions of 4.10 apply. Additional use of the band by
      a service that is not covered by the same provision of 4.10, ie: a non-safety service is not feasible.
                                              -5-                    ACP-WGF24/WP-09


            The ACP WG-F is invited to:
       Note the information and conclusion of this paper
       Use the information provided as necessary in national preparation meetings to ensure
        protection of the current use of the band