Tail Missing Feedback
Applicable alarms (4)
Likely outcomes — click to expand
Based on 656,635 missions where this alarm fired in the last
90 days
(for TAIL_STBD_MISSING_FEEDBACK) as of 2026-10-13
Grounding vs discrepancy (of 2,993 ZERP issues filed on these missions)
Top work orders
- EV2: Inspect Aircraft after Droid Left Behind (DLB) · MAINTENANCE_ACTION 234
- EV2: Replace Aft Motor Assembly on Aircraft · MAINTENANCE_ACTION 87
- EV2: Troubleshoot Electrical Contacts on Aircraft and Droid · INSPECTION 44
- EV2: Replace Flight Computer on Aircraft · MAINTENANCE_ACTION 41
- Troubleshoot Hover Motor Resistance on Aircraft · INSPECTION 40
Frequently replaced parts
| Part | P/N | Removals |
|---|---|---|
| SERVO, ASSY, UNIVERSAL, P2 | 20398 | 959 |
| CW/CCW HOVER MOTOR ASSY, ZIP, P2 | 19607 | 682 |
| AFT MOTOR ASSY, EV.1 REWORK, ZIP, P2 | 23548 | 609 |
| TAIL ASSY STBD(RH), PRE-EV2, ZIP, P2 | 21840 | 604 |
| TAIL PORT, ZIP, P2 | 31280 | 575 |
Aggregated from fleet operations and refreshed daily. Individual outcomes may vary by site and configuration.
Background
Section titled “Background”The GNC Zip Controller faulted a tail servo because expected feedback messages were missing. The source troubleshooting treats non-rotor actuators as servo feedback paths.
Troubleshooting Steps
Section titled “Troubleshooting Steps”graph TD
check-rs485-comms["<small><b>Step 1</b></small><br/>Telemetry: Check whether the affected tail servo..."]
follow-servo-rs485("<small><b>Step 2</b></small><br/>Reference: Poor servo communication is present. ...")
check-companion-alarms{"<small><b>Step 3</b></small><br/>Question: Check whether either of these Exec wa..."}
follow-motor-control-guide("<small><b>Step 4</b></small><br/>Reference: A companion Exec watchdog alarm was s...")
escalate-firmware-platform{{"<small><b>Step 5</b></small><br/>Escalation: Escalate to P2 Firmware Platform On C..."}}
check-rs485-comms -->|Yes, poor comms are present| follow-servo-rs485
check-rs485-comms -->|No, poor comms are not present| check-companion-alarms
check-companion-alarms -->|Yes, one or more listed ala...| follow-motor-control-guide
check-companion-alarms -->|No listed alarms were set| escalate-firmware-platform
style follow-servo-rs485 fill:#f3e8ff,stroke:#a855f7,color:#581c87
style check-companion-alarms fill:#dbeafe,stroke:#3b82f6,color:#1e3a5f
style follow-motor-control-guide fill:#f3e8ff,stroke:#a855f7,color:#581c87
style escalate-firmware-platform fill:#fef3c7,stroke:#f59e0b,color:#78350f Hover or focus a block to preview it. Select the block to keep the preview open.
Load the plot below and look only at the tail servo named by the active alarm. If the alarm is for the port tail servo, use the port tail servo traces. If the alarm is for the starboard tail servo, use the starboard tail servo traces. A spike on a different servo does not explain this alarm.
Look for signs that messages from the affected servo were being missed or corrupted. Poor comms are present if either of these is true outside a power cycle:
- The message counter jump, shown as
message_sequence_number(delta), goes above 1.- The RX frame error count goes above 10.
Ignore jumps that happen during an aircraft power cycle. A power cycle usually looks like several signals resetting at the same time, and Show Alarms should show power-cycle or reboot activity nearby. If the message counter goes negative, use Show Alarms to zoom in on the part of the mission near this missing-feedback alarm before deciding.
Representative poor-comms example:
- MOTOR_CONTROL_FEEDBACK_SERVO_TAIL_PORT_FWD_UNAVAILABLE
- MOTOR_CONTROL_FEEDBACK_SERVO_TAIL_STBD_FWD_UNAVAILABLE
Resolution note
This plaintext block is ready to paste into the Resolution Note field on the ZERP issue you're working. The permalink below gives engineering review access to the full step-by-step timeline including any back-navigation; share it if the case needs a deeper look.
Permalink:
Session is too large to fit in a URL. Use Download session JSON to share the timeline out-of-band.
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