High Vibration
Vibration is the direct readout of rotating-part imbalance (article 14). Among the exceedance alerts this one has the most negotiable temperament — the source text says outright that it is mainly a prompt to monitor the parameters more carefully, and alone does not necessarily require a shutdown. The diagnostic key is which shaft: high N1 vibration has two possible diseases (icing, curable, versus mechanical, manageable only by slowing down), while high N2/N3 vibration has essentially one handling (slow down). The ice-shedding dance — idle-then-increase, repeated — is this article's signature technique.
1. Trigger and diagnosis
"The EIVMU detects abnormal high vibrations of: ‐ The LP rotor (N1 vibrations), or ‐ The IP rotor (N2 vibrations), or ‐ The HP rotor (N3 vibrations). This alert triggers when: ‐ N1 is greater than 3.3 units, or ‐ N2 is greater than 2.6 units, or ‐ N3 is greater than 4.0 units."
The EIVMU — owner of the narrow-band tracking described in article 14 — watches all three rotors. Converting at that article's calibration (1 unit = 0.3 ips): N1 0.99 / N2 0.78 / N3 1.2 ips. The three shafts get different tolerances (synthesis): N2 is the most sensitive — an IP-rotor imbalance transmits most directly through the intermediate case — while the fan shaft, biggest and heaviest, is allowed the largest absolute amplitude.
The alert's character, in the source's own words:
"This alert is mainly an indication for the flight crew to monitor the engine parameters more carefully. This alert alone does not necessarily require engine shutdown."
And the circumstantial evidence that usually rides along:
"High N1, N2 or N3 vibrations are usually associated with: ‐ Airframe vibrations, and ‐ Cockpit and cabin smoke, and/or the smell of burning. This may be due to the contact of the tip of the compressor blade with the associated seals. In the case of high N1 vibrations, another cause may be the icing of the fan blades and/or spinner."
The link between a burning smell in the cabin and engine vibration is startling but sound (synthesis): blade tips rubbing their abradable seal liners (article 01) shed particles into the bleed air — bleed feeds the air conditioning — the cabin smells it. Vibration plus burning smell does not equal a cabin fire: check the VIB readings first — this is the cross-check point between this procedure and the smoke procedures. And N1's exclusive ice diagnosis comes from geography: the fan and spinner are the only rotating parts in the whole engine that face ice crystals directly — the reverse scenario of the minimum-N1 anti-ice floor in article 05.
2. The handling fork
During takeoff: crew awareness — the takeoff is not interrupted (the V1 philosophy extended). In other phases, fork by shaft:
High N1, no icing suspected. Lever BELOW LIMIT — slow the rotor, lower the vibration. A mechanical imbalance cannot be cured in flight, only lived with: find the practical speed where vibration is least (the same clause as the sustained-vibration strategy in article 25).
High N1, icing suspected — the shedding dance:
"A/THR OFF / ENG (AFFECTED) ANTI ICE ON / ENG (AFFECTED) IDLE THEN INCRS — REPEAT AS NECESSARY — To shed ice, it may be necessary to perform several engine thrust variations. During each engine run-up, increase thrust up to an appropriate setting for the flight phase. After each engine run-up, the vibrations should decrease indicating the progress of the ice shedding. When the ice is shed, the vibrations should return to normal level and the flight crew can resume normal engine operation."
The dance's physics (synthesis): idle gives the anti-ice heat time to soften the ice at its root (low rotation, low aerodynamic load, heat soaking through); the brisk run-up uses centrifugal force to throw the loosened ice off (into the bypass duct, harmlessly). The vibration value is your scale: every round that ends lower means another piece gone. The QRH adds two disciplines: one engine at a time, and if the anti-ice wasn't already on, switch it on at idle fan speed, with about 30 seconds between engines — two engines dancing together means two thrust sources fluctuating together; the 30-second offset guarantees one stable engine at every moment (the same philosophy as the one-lever-at-a-time rule in article 31). If vibration stays high after the dance: back to the BELOW LIMIT coexistence line. A note adds that fan icing can also cause a temporary EGT rise — ice-disturbed flow costs efficiency.
High N2 / N3. Straight to BELOW LIMIT — ice cannot reach the core rotors, so core vibration has only mechanical explanations (blade damage, imbalance, bearings — the oil-film and chip-detector territory of article 10). Slow down, coexist, inspect on the ground. And the QRH's landing clause: "After landing, if vibrations continue: SHUT DOWN ENGINE WHEN POSSIBLE" — don't let a damaged rotor turn one minute longer than it must.
3. Scenario walk-throughs
After a winter cloud penetration, N1 VIB climbs to 3.6. Suspect ice, run the dance; two rounds later it reads 1.2 — resume normal operation, and log the event.
Cruise, N3 VIB 4.2, cabin reports a burning smell. Slow the rotor and coexist; brief the cabin crew that this is observe-first, not fire; after landing, maintenance checks the chip detectors (article 10) and bores the scope.
Both engines' VIB creeping up in icing conditions. Dance one at a time, 30 seconds apart — with the autothrust off, your hands keep the total thrust symmetric.
Still above 4 units while taxiing in. Shut it down when possible and take the tow — don't ride it to the gate.
Self-test
[!note]- Q1. The three trigger thresholds, and in ips? N1 > 3.3 / N2 > 2.6 / N3 > 4.0 units — at 0.3 ips per unit: 0.99 / 0.78 / 1.2 ips. N2 is the most sensitive shaft.
[!note]- Q2. Cabin reports a burning smell and that engine's VIB is elevated — first explanation? Blade tips rubbing the seal liners, particles riding the bleed air into the air conditioning — vibration and smell from one source. Cross-check with the smoke procedures; don't rush to declare a cabin fire.
[!note]- Q3. How do you know the shedding dance is working? Vibration should decrease after each run-up — the VIB value is the progress bar. Back to normal level = resume normal operation.
[!note]- Q4. Why is there no icing branch for N2/N3? Ice can only reach the fan and spinner — the N1 domain. Core-rotor vibration has only mechanical explanations: slow down, coexist, inspect on the ground.
[!note]- Q5. Both engines suspected of icing — how is the dance sequenced? One engine at a time; anti-ice not yet on goes on at idle fan speed, about 30 seconds apart — so that at any moment one engine holds stable thrust.
Key takeaways
| Topic | Essentials |
|---|---|
| Thresholds | N1 3.3 / N2 2.6 / N3 4.0 units (× 0.3 ips); N2 most sensitive |
| Character | a monitoring prompt — alone, not a shutdown order; takeoff phase: awareness only |
| Circumstantial evidence | airframe vibration; cabin smoke/burning smell = tip-rub particles via bleed air — not necessarily fire |
| N1 fork | no ice → lever below limit, coexist · ice suspected → A/THR off, anti-ice on, idle-then-increase, repeat; VIB is the progress bar |
| Discipline | one engine at a time; 30 s offset between engines; ice shedding may transiently raise EGT |
| N2/N3 | mechanical only — below limit, inspect; still vibrating after landing → shut down when possible |
References
- FCOM PRO (engine abnormal procedures: HIGH VIBRATIONS complete section — thresholds, monitoring character, associated-evidence note, branch handling, the full shedding passage) — quoted verbatim.
- QRH (high engine vibration) — the one-at-a-time discipline, the idle-fan-speed/30-second anti-ice sequencing, the shut-down-after-landing clause.
- Integrative synthesis (marked in text): the per-shaft tolerance reading; the smell-propagation chain; the soften-then-sling physics of the dance; the symmetric-thrust logic of the 30-second offset.
Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.