UNRELIABLE AIRSPEED: DIAGNOSE INFORMATION, NOT A REASSURING DISPLAY
Distinguish an explicitly failed source from a source that continues to produce plausible but erroneous information. The FCTM describes cases in which airspeed or altitude appears normal without a corresponding detected fault. A discrepancy between sources is important evidence, but agreement is not conclusive: two similarly affected sources can agree and cause a correct source to be rejected. Crosscheck the relationship between speed, attitude, thrust and flight path using the applicable procedure. A majority vote by eye is not a complete unreliable-air-data diagnosis. [21, PDF pp. 255–258]
When the safe conduct of flight is affected, use the applicable unreliable-speed immediate actions and subsequent procedure. This chapter does not create a substitute pitch/thrust table or memory-item set. The source distinguishes initial stabilisation from later troubleshooting and from continued flight on appropriate references. The required values depend on the aircraft, phase, mass, altitude and configuration. Keeping a generic initial setting indefinitely can create a different speed problem after the immediate situation has been stabilised. Identify the correct table and transition before a planned exercise. [21, PDF pp. 257–260]
Air-data and inertial-reference functions can fail independently. Follow the specified control when isolating an ADR: the FCTM explicitly distinguishes its pushbutton from the rotary selector, which also removes the inertial-reference supply. Removing otherwise usable attitude/navigation information while investigating speed can substantially change the problem. Use the installed panel's actual unit numbers and source-switching instructions, particularly where the physical order is not 1–2–3. The aim is to isolate the identified information fault while preserving references that remain valid. [21, PDF p. 255] [65, PDF pp. 6–8]
GPS groundspeed and altitude can support a wider trajectory crosscheck when available; they are not interchangeable with indicated airspeed or a pressure-altitude clearance reference. Wind and the measurement reference matter. The source also warns that unreliable altitude information can affect vertical speed and the flight-path vector. Therefore, do not declare an indication independent simply because it appears on another page. Establish its data source and what question it can answer. Fenix documents a configurable backup-speed-scale option; its presence alone does not validate every activation condition or replace the applicable unreliable-speed procedure. [21, PDF p. 260] [70, fnx_buss_enabled; documented option scope]
A useful lesson introduces one identified air-data fault from a known state before progressing to more ambiguous combinations. Record the affected channels, active law, AP/A/THR state, displayed indications and the actual aircraft path. Use the applicable current Fenix document for the selected airframe. The 24–25 October 2026 developer snapshot identifies a dynamic unreliable-speed table as A320-only; that finding must retain its date and applicability. A familiar family cockpit does not establish that the same table is valid for an A319 or A321. [21, PDF pp. 255–260] [83, Dynamic unreliable-speed document identified as A320-only in dated snapshot]