Date: 201211
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Flight conditions: VMC
Light: Night
Aircraft: DA20 Undifferentiated
Operator: FBO
Operating under: Part 91
Flight plan: None
Mission: Personal
Flight phase: Final Approach
ATC / advisory: TRACON ZZZ
Aircraft component: Intake Assembly
Component problem: Malfunctioning
Reporter organization: FBO
Reporter function: Single Pilot; Pilot Flying
Reporter qualification: Flight Crew Private
Second reporter function: Technician
Anomaly: Aircraft Equipment Problem Critical; Inflight Event / Encounter Other / Unknown
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Inflight Shutdown; General Maintenance Action
Contributing factors: Aircraft; Procedure; Manuals; Human Factors
Primary problem: Manuals

Narrative: I was on the last leg of a VFR cross country at night. Approach cleared me into the Class B for the transition to ZZZ; where the FBO is located. Upon clearing me into the Class B; the Approach Controller also descended me to 4;000 out of 4;500. As I eased the throttle lever back in order to descend the aircraft; I noted that the engine RPM indicator did not show a decrease. I thought this may simply be due to the fact that the aircraft was descending and the prop was keeping engine RPM's up. I realized as soon as I leveled off at 4;000 that this was not the case; as the RPM's were stuck around 2;700 in level flight; no matter what position I moved the throttle lever into. After fiddling with the lever for a minute or so with no success; I explained the problem to Approach. They indicated their readiness to help; before handing me off to another Approach Controller who was aware of the situation. I explained to this Controller that I would need to descend early so that I could level off and dissipate some of the high speed that I would naturally accrue in a full-power descent. The Controller gave me a vector to intercept the final approach course into ZZZ and asked for fuel and souls aboard. The airport was easy to spot with all the fire trucks at night. The Controller handed me off to Tower. I leveled off at about 1;000 AGL and turned final. I judged that I should cut the mixture off about 2 miles out at 1;000 AGL; which would allow the speed to dissipate to roughly 73 KIAS for the landing. I had a GPS distance to the center of the field and also asked the Tower Controller to call my 2 NM from threshold on final. Upon reaching what I assessed was about 1.8 NM from the threshold; I pulled the mixture and engine power ceased immediately. The aircraft slowed rapidly and it became clear within a few seconds that I would not reach the threshold of the runway. I adjusted my plan for a soft field landing on the airport grass short and left of the threshold. I moved laterally left about 100 FT so as not to land on the rabbit. What ensued was a routine soft field landing with very aggressive braking. Once stopped; I pulled the emergency fuel shut-off valve; killed all electrics; and evacuated the aircraft. No damage was done to the aircraft in the course of the flight; approach; or landing as a result of the malfunctioning throttle.


Second narrative: On final the pilot reported no throttle response at 'Full Open' and elected to secure the engine via the mixture control to complete his landing. He landed short of the runway in the grassy area with no damage to the aircraft. A post landing inspection revealed that the throttle control arm had spun on the air/throttle shaft. This Mechanic completed an Annual Inspection to the airframe and engine two months prior. This Mechanic will attest that at the time of the inspection; the control arm was secure and that no play was detected between the throttle cable rod end and the control arm; or between the control arm and the throttle body shaft. At the Annual Inspection; this Mechanic physically rotated the control arm to check that the stops were hitting correctly and rigged; that the throttle cable moved freely; and that there was no bearing play between the throttle cable rod end and the control arm. Post Annual [Inspection] ground runs and a maintenance check flight were performed by this Mechanic with no performance problems noted. As far as this Mechanic knows the throttle control arm has never been adjusted or removed. This Mechanic believes the self-locking nut may have lost its torque or was improperly installed at [engine] factory.When the control arm was removed from the throttle shaft; Visual inspection confirmed that the shaft had indeed rotated within the throttle control arm. The shaft serrations were deformed. The serrations on the control arm appeared normal; but there was not another control arm available to compare it to. It should also be noted that there is only about an 1/8' inch of serrations on the throttle shaft that contacts the throttle control arm and that the serrations in the control arm are coned. If the self-locking nut loses torque and relaxes even one thread length; the integrity of the system is compromised. At the least there should be a requirement to torque check this nut at every 50-hour Inspection. A better action would be the installation of a locking device such as a nut/cotter pin combination incorporated into throttle shaft; and the best method would be to completely spline the shaft and control arm and add a stronger safety device. The IO-240 Maintenance Manual says to check the security of the throttle body but not how. Physically touching it with hands; rotating; or pulling to check for looseness is not the best way to check for security of this component. I urge all mechanics working on Diamond DA 20 aircraft with IO-240 engines to put a torque wrench on the throttle control arm shaft nut to ensure the shaft is properly seated. The engine manufacturer was notified and a new throttle body ordered.

Second callback: Reporter stated the engine manufacturer's representative had informed him about similar incidents where the throttle control arm had spun at the lower attachment to the spring loaded air/throttle body shaft; allowing the throttle to go to 'Full Open' position in flight. And that was exactly what occurred on the Diamond DA-20 aircraft on which he had previously accomplished an Annual Inspection. A Canadian Investigator he had spoken with later noted the metal material of the throttle control arm had been changed to stainless steel; which is a stronger metal; but the throttle body shaft was still made of a softer material.Reporter stated the connections of the throttle control arm lower attachment to the air/throttle body shaft is a critical fit. But the serrations on the body shaft are only one-eighth of an inch long that actually contact the serrations in the coned shaped throttle control arm. Any relaxation of the locking nut that secures the assembly; even backing-off by one thread; will allow slippage of the control arm tension. The IO-240 engine comes from the manufacturer with the push/pull throttle cable installed on the engine and connections already secured at the air/throttle body shaft to the throttle control arm. Reporter stated either the torque had backed-off on the locking nut which may have a fiber center for the locking feature instead of an all metal locking nut; or the Throttle Control arm serrations and the Throttle shaft serrations were not meshed; not properly aligned; and became stripped or later re-aligned and the tension on the locking nut reduced. On the incident aircraft he had previously serviced; the Maintenance Manual (M/M) did not require a Torque Check of the locking nut. But the post incident inspection noted the Throttle Body shaft serrations were found deformed; even though the Control Arm serrations appeared normal. Looseness or stripped serrations are effectively NOT allowing control of the air intake.

Synopsis: DA20 pilot reported having no throttle response on final approach; and since the throttle was at 'Full Open' he elected to secure the engine via the mixture control to complete his landing. He landed short of the runway in the grassy area with no damage to the aircraft. A Maintenance Technician reported a post landing inspection revealed that the throttle control arm had spun on the air/throttle shaft.