PERF DES AND THE DESCENT PROFILE: STUDY MODE AND MODEL TOGETHER
PERF DES presents the descent speed schedule and predictions, while the actual FMA establishes how the aircraft is being guided. Managed DES and open descent solve different tasks. V3's revised trajectory can include idle segments, geometric segments, decelerations and level portions. A level segment therefore needs interpretation against the constraints and energy plan before being diagnosed as a frozen descent. [66, PDF pp. 205,211-240,670] [21, PDF pp. 141-146] [82]
1. Complete the input chain. Review the active arrival geometry and restrictions, cruise reference, mass, forecast winds and destination preparation before judging T/D. A route shortcut or changed descent speed can move the path materially. Compare active data, not an uninserted TMPY or stale SEC alternative. Prediction study is only meaningful once the intended route and assumptions are identified. [66, PDF pp. 205,211-240,670] [21, PDF pp. 141-146]
2. Read ACT MODE and the speed schedule. Inspect managed versus selected speed and any pre-entered descent schedule. The source's combined Mach/speed syntax uses a dash, for example the descriptive form .MM-KKK; substitute actual planned values rather than typing the descriptive letters. If a value is rejected, check field grammar and flight phase before assuming the aircraft cannot accept a descent-speed revision. [66, PDF pp. 205,211-240,670] [21, PDF pp. 141-146]
3. Keep prediction-to-altitude separate from clearance. The source can calculate time/distance to an altitude displayed on PERF DES. An overwritten study value does not change the FCU or authorise descent. Set the actual clearance appropriately through the normal guidance workflow and verify DES versus OP DES on the FMA. A printed altitude restriction can remain visible while the selected vertical mode does not manage it. [66, PDF pp. 205,211-240,670] [21, PDF pp. 141-146]
4. Interpret the path and energy cues. Read vertical deviation, upcoming restrictions, deceleration segments and any drag/steep-path indication together. Check whether the aircraft has the distance and configuration expected by the profile. Avoid deleting valid altitude or speed constraints solely to make the predicted line easier. The operational response must preserve clearance and configuration limits while addressing the actual energy condition. [66, PDF pp. 205,211-240,670] [21, PDF pp. 141-146]
5. Investigate a non-computation methodically. Distinguish incomplete prerequisites, an uninserted/invalid route state and a build-specific prediction issue. The dated V3 known-issues snapshot includes NO-COMPUTATION cases; it does not establish a universal reset cure. Record the exact active route, phase, mass and wind state. A reproducible narrow case is more useful than a series of unrelated edits that remove the evidence. [66, PDF pp. 205,211-240,670] [21, PDF pp. 141-146]