High-Lift Faults — WTB, Asymmetry, Runaway and Disconnect
A shafted transmission has two catastrophic failure modes — runaway (a surface moving uncommanded) and asymmetry (left and right wings differing) — and the wing-tip brakes (WTB) are the hard stop that locks the transmission against them. Crucially, a WTB cannot be released in flight: once it locks, that surface is frozen at its current position. This article covers what the SFCCs monitor, what the WTB does, and the disconnect protection.
The SFCC monitor the power transmission system for these failure conditions: asymmetry (a position difference between the two APPUs), runaway (a position difference between the APPUs and the FPPU), uncommanded movement, overspeed, flap disconnect... — AMM 27-51-00
1. What the SFCCs monitor
Per AMM 27-51-00, the two SFCCs continually monitor the transmission for:
| Fault | Detected by |
|---|---|
| Asymmetry | a position difference between the two APPUs (left vs right wing) |
| Runaway | a position difference between the APPUs and the FPPU (surface moved, command didn't) |
| Uncommanded movement | movement in the wrong direction, or away from the last commanded position |
| Overspeed | surface vs speed |
| Flap disconnect | excessive inner/outer flap differential movement |
| Track failure | track-4 sensors |
[!note]- Asymmetry vs runaway — two different sensor disagreements (integrative synthesis) The detection logic is elegant: asymmetry is the two APPUs (one per wing) disagreeing — the wings are at different positions; runaway is the APPUs vs the FPPU disagreeing — the surfaces moved but the commanded-position feedback didn't (25). Both threaten control (asymmetry → roll; runaway → unwanted configuration/overspeed), so both trip the WTB.
2. The wing-tip brakes — locked, and not releasable in flight
Per FCOM DSC-27-30-10 / AMM 27-51-00, on these failures the WTB stops and locks the transmission. Each WTB is electrohydraulic (slats blue+green, flaps green+yellow), with one solenoid per SFCC; de-energising the solenoids locks the brake. The EWD shows amber F LOCKED (flap WTB) or S LOCKED (slat WTB).
[!warning]- A WTB cannot be released in flight — you keep the configuration you have The defining operational fact: the WTBs cannot be released in flight (FCOM DSC-27-30-10). Once a WTB locks the slats or flaps, that group is frozen at its current position for the rest of the flight — reset is only on the ground (via CMS, one slat/flap lever cycle, or an SFCC reset). So a high-lift runaway/asymmetry commits you to that configuration, and the approach/landing must be flown and the speeds respected for whatever is locked.
[!note]- Flaps and slats lock independently (integrative synthesis) Because slats and flaps have separate transmissions and separate WTBs, a flap WTB still allows slat operation, and a slat WTB still allows flap operation (FCOM DSC-27-30-10, 25). So an S LOCKED with flaps free, or F LOCKED with slats free, is the normal pattern — you lose one group, not both.
3. Flap disconnect protection
Per AMM 27-50-00/51-00, the flap disconnect detection system measures excessive differential movement between the inner and outer flaps (via disconnect proximity switches on the interconnecting struts) and inhibits flap operation to prevent further damage — guarding against an attachment failure letting one flap section move independently.
[!warning]- Disconnect detection stops the flaps to prevent a split flap If a flap attachment fails, the inner and outer flaps could move apart (a "split" flap → asymmetric lift). The disconnect system detects the excessive differential and inhibits flap operation (AMM 27-50-00) before that develops — a separate protection from the WTB asymmetry (which is left-vs-right; disconnect is inner-vs-outer on the same wing).
4. Degradations recap and warning level
Per FCOM DSC-27-30-10, the non-locking degradations: one SFCC inoperative → both slats and flaps half speed; one hydraulic system inoperative → the affected group half speed. All high-lift failures are level-2 cautions — never a level-3 warning — shown on the EWD.
5. Counterintuitive points
[!warning]- WTB locks cannot be released in flight A locked slat/flap group stays put until a ground reset (FCOM DSC-27-30-10) — plan the landing for it.
[!warning]- Asymmetry is left-vs-right; disconnect is inner-vs-outer Asymmetry (APPU vs APPU) trips the WTB; flap disconnect (inner/outer differential) inhibits the flaps — different protections (AMM 27-51-00 / 27-50-00).
Self-test
[!note]- Q1. What does the SFCC monitor, and how are asymmetry and runaway distinguished? Asymmetry, runaway, uncommanded movement, overspeed, disconnect, track. Asymmetry = two APPUs disagree (L/R); runaway = APPUs vs FPPU disagree.
[!note]- Q2. What does the WTB do, and can it be released in flight? Stops and locks the transmission; cannot be released in flight (ground reset only: CMS / lever cycle / SFCC reset). EWD F LOCKED / S LOCKED (amber).
[!note]- Q3. Do flaps and slats lock together? No — separate transmissions/WTBs; flap WTB still allows slats and vice versa.
[!note]- Q4. What does flap disconnect detection do? Detects excessive inner/outer flap differential and inhibits flap operation (prevents a split flap).
Key takeaways
| Point | Detail |
|---|---|
| Monitored faults | asymmetry (APPU≠APPU), runaway (APPU≠FPPU), uncommanded movement, overspeed, disconnect, track |
| WTB | locks the transmission; not releasable in flight; ground reset (CMS/lever cycle/SFCC) |
| WTB hydraulics | slats blue+green, flaps green+yellow; one solenoid per SFCC |
| Independence | flap WTB allows slats; slat WTB allows flaps |
| Disconnect | inner/outer flap differential → inhibits flaps (split-flap protection) |
| Degradation | 1 SFCC → both half speed; 1 hydraulic → affected group half speed |
| Warning | level-2 cautions only; EWD F/S LOCKED (amber) |
References
- AMM 27-51-00 (Flaps Electrical Control and Monitoring) — SFCCs monitor for asymmetry (position difference between the two APPUs), runaway (position difference between APPUs and FPPU), uncommanded movement (wrong direction or away from last commanded position), overspeed, flap disconnect, track failure; level-2 cautions only; WTB control (slats blue+green / flaps green+yellow, one solenoid per SFCC, de-energise locks, reset via CMS / lever cycle / SFCC reset, manual release for maintenance).
- AMM 27-50-00 (Flaps) — flap disconnect detection (excessive inner/outer differential movement) inhibits flap operation.
- FCOM DSC-27-30-10 (Flaps and Slats) — WTBs activate on runaway/asymmetry/overspeed, not releasable in flight; flap WTB still allows slats and vice versa; one SFCC inop → both half speed, one hydraulic inop → affected group half speed; amber F LOCKED / S LOCKED on EWD.
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.