AD 2022-04-05 Prompted By A Determination That Radio Altimeters Cannot Be Relied Upon To Perform Their Intended Function...

The FAA is adopting a new airworthiness directive (AD) for all The Boeing Company Model 757 airplanes and Model 767 airplanes.

This AD was prompted by a determination that radio altimeters cannot be relied upon to perform their intended function if they experience interference from wireless broadband operations in the 3.7-3.98 GHz frequency band (5G C-Band), and a recent determination that, during approach, landings, and go-arounds, as a result of this interference, certain airplane systems may not properly function, resulting in increased flightcrew workload while on approach with the flight director, autothrottle, or autopilot engaged. The FAA is issuing this AD to address 5G C-Band interference that could result in increased flightcrew workload and could lead to reduced ability of the flightcrew to maintain safe flight and landing of the airplane. This AD requires revising the limitations and operating procedures sections of the existing airplane flight manual (AFM) to incorporate specific operating procedures for landing distance calculations, instrument landing system (ILS) approaches, non-precision approaches, speedbrake deployment, and go-around and missed approaches, when in the presence of 5G C-Band interference as identified by Notices to Air Missions (NOTAMs). The FAA is issuing this AD to address the unsafe condition on these products. This AD is effective February 14, 2022.

Supplementary Information: In March 2020, the United States Federal Communications Commission (FCC) adopted final rules authorizing flexible use of the 3.7-3.98 GHz band for next generation services, including 5G and other advanced spectrum-based services.1 Pursuant to these rules, C-Band wireless broadband deployment was permitted to occur in phases with the opportunity for operations in the lower 0.1 GHz of the band (3.7-3.8 GHz) in certain markets beginning on January 19, 2022. This AD refers to “5G C-Band” interference, but wireless broadband technologies, other than 5G, may use the same frequency band. These other uses of the same frequency band are within the scope of this AD since they would introduce the same risk of radio altimeter interference as 5G C-Band.

The radio altimeter is an important aircraft instrument, and its intended function is to provide direct height-above-terrain/water information to a variety of aircraft systems. Commercial aviation radio altimeters operate in the 4.2-4.4 GHz band, which is separated by 0.22 GHz from the C-Band telecommunication systems in the 3.7-3.98 GHz band. The radio altimeter is more precise than a barometric altimeter and for that reason is used where aircraft height over the ground needs to be precisely measured, such as autoland, manual landings, or other low altitude operations. The  receiver on the radio altimeter is typically highly accurate, however it may deliver erroneous results in the presence of out-of-band radio frequency emissions from other frequency bands. The radio altimeter must detect faint signals reflected off the ground to measure altitude, in a manner similar to radar. Out-of-band signals could significantly degrade radio altimeter functions during critical phases of flight, if the altimeter is unable to sufficiently reject those signals.

The FAA issued AD 2021-23-12, Amendment 39-21810 (86 FR 69984, December 9, 2021) (AD 2021-23-12) to address the effect of 5G C-Band interference on all transport and commuter category airplanes equipped with a radio (also known as radar) altimeter. AD 2021-23-12 requires revising the limitations section of the existing AFM to incorporate limitations prohibiting certain operations, which require radio altimeter data to land in low visibility conditions, when in the presence of 5G C-Band interference as identified by NOTAM. The FAA issued AD 2021-23-12 because radio altimeter anomalies that are undetected by the automation or pilot, particularly close to the ground (e.g., landing flare), could lead to loss of continued safe flight and landing.

Since the FAA issued AD 2021-23-12, Boeing has continued to evaluate potential 5G C-Band interference on aircraft systems that rely on radio altimeter inputs. Boeing issued Boeing Multi Operator Message MOM-MOM-22-0022-01B(R2), dated February 1, 2022; Boeing Flight Crew Operations Manual Bulletin TBC-67 R1, “Radio Altimeter Anomalies due to 5G C-Band Wireless Broadband Interference in the United States,” dated February 1, 2022; Boeing Flight Crew Operations Manual Bulletin TBCC-72 R1, “Radio Altimeter Anomalies due to 5G C-Band Wireless Broadband Interference in the United States,” dated February 1, 2022; Boeing Flight Crew Operations Manual Bulletin TBC-86 R1, “Radio Altimeter Anomalies due to 5G C-Band Wireless Broadband Interference in the United States,” dated February 1, 2022; and Boeing Flight Crew Operations Manual Bulletin TBC4-33 R1, “Radio Altimeter Anomalies due to 5G C-Band Wireless Broadband Interference in the United States,” dated February 1, 2022; for Model 757 and 767 airplanes.

Based on Boeing's data, the FAA identified an additional hazard presented by 5G C-Band interference on The Boeing Company Model 757 and 767 airplanes. The FAA determined anomalies due to 5G C-Band interference may affect multiple other airplane systems using radio altimeter data, regardless of the approach type or weather. These anomalies may not be evident until very low altitudes. Impacted systems include, but are not limited to, autopilot flight director system; autothrottle system; flight controls; flight instruments; traffic alert and collision avoidance system (TCAS); ground proximity warning system (GPWS); and configuration warnings.

In the event of 5G C-Band interference, landing performance and flightcrew workload can be adversely impacted.

This may have multiple effects, including:

• Autopilot Flight Director System: NO AUTOLAND Autopilot Status Annunciation may be shown; autopilot may not engage; autopilot disconnect may occur when LAND 2 or LAND 3 status is shown; the flight directors may provide erroneous guidance during ILS approaches; autoland runway alignment may not occur or may activate earlier or later than expected; autoland flare may not occur, however, FLARE mode can be erroneously annunciated on the FMA (flight mode annunciation); or go-around mode may not be available.
• Autothrottle System: Autothrottle can remain in SPD (speed) mode and may advance to maintain speed during flare instead of reducing the thrust to IDLE; or autothrottle may retard to idle prematurely in the flare.
• Flight Controls: Automatic speedbrake deployment may not occur after touchdown (for Model 757 and 767 models with Yaw Damper Stabilizer Trim module (YSM)); or SPEEDBRAKES EXT Caution message may not be available.
• Flight Instruments: The RA (radio altimeter) indication may not be shown; the RADIO minimums indications (flashing or turning amber) may not be shown or may be erroneous; the rising runway symbol may not be shown; the localizer deviation alert amber scale and flashing pointer may not be shown (deviation indications are still available); or the glideslope deviation alert amber scale and flashing pointer may not be shown (deviation indications are still available).
• TCAS: TCAS alerts may not be available (TCAS alerts that do occur will be valid); or TCAS inhibits for resolution advisories may be erroneous.
• GPWS: GPWS alerts may not be available or may be erroneous (although look-ahead terrain alerting remains available); radio altimeter-based altitude and minimums aural callouts during approach may not be available or erroneous; or windshear detection systems (predictive and reactive) may be inoperative.
• Configuration Warnings: Erroneous landing gear configuration warning may occur.
• Considerations for Dispatch: For Model 757 and 767 airplanes with YSM, adjust operational (time of arrival) landing distance for manual speedbrakes. For airplanes without YSM, no impacts on dispatch landing performance calculations.
• Other simultaneous flight deck effects associated with the 5G C-Band interference could increase pilot workload.

These effects may cause erroneous indications and annunciations, as well as conflicting information, being provided to the flightcrew during a critical phase of flight. This could lead to reduced ability of the flightcrew to maintain safe flight and landing of the airplane, and is an unsafe condition.

To address this unsafe condition, this AD mandates procedures for operators to incorporate specific operating procedures for landing distance calculations, ILS approaches, non-precision approaches, speedbrake deployment, and go-around and missed approaches, when in the presence of 5G C-Band interference as identified by NOTAMs.

The FAA notes that for those airplanes equipped with YSM, the 5G interference to the radio altimeter creates an error to the YSM which causes the speedbrakes to not automatically deploy on landing; the flightcrew must manually deploy the speedbrakes when this occurs. Further, the additional landing distance calculation is required due to the differences in manual deployment versus automatic deployment during landing.

Finally, the FAA notes that AD 2021-23-12 remains in effect and thus prohibits certain ILS approaches. Thus, this AD addresses procedures applicable only to those ILS approaches not prohibited by AD 2021-23-12.

The FAA is issuing this AD to address the unsafe condition on these products.

FMI: www.regulations.gov