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SB003 Alta X – Motor Stall on Fast Vertical Descent (7/10/20)

Date Released: July 10th 2020

Subject: Alta X Motor Stall on Fast Vertical Descents

Effectivity: All Alta X

Require Action: Modify flight envelope (reduce vertical velocities)

INTRODUCTION

This service bulletin is being issued to alert all Alta X pilots of an in-flight failure that a Freefly test pilot experienced on July 9th 2020.

BACKGROUND

During flight testing a Freefly test pilot executed an aggressive maneuver in manual mode that involved climbing to ~400ft while initiating a zero throttle zero horizontal speed descent.  The pilot let the Alta X fall at zero commanded throttle for 4 seconds.  At the end of the 4 second fall the Alta X was traveling vertically at  ~ 16m/s (35mph).  At this point Motor 1 (front right) stalled and shut down and the Alta X crashed.  The Alta X was equipped with a 20lb payload for this test.

Freefly replicated the issue by testing on our ground test setup (truck with Alta X motor and prop mounted) and was able to replicate the issue at speeds of 20m/s to 22.5 m/s (45 to 50mph)

Video from the ground test

DISCUSSION

Some key points to convey regarding this issue:

  • This issue has only been observed in a zero throttle straight down descent
  • We will be characterizing this failure further in the upcoming weeks to issue better guidance
  • Altitude / Position modes are limited to 4m/s descent speed and will have no issue

RECOMMENDATION

  1. Do not execute zero throttle descents in manual mode
  2. If you need to execute zero throttle descents in manual mode, do not execute zero throttle straight down descents that achieve velocities faster than 13m/s (30mph)

Freefly will be issuing additional updates as our investigation continues

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