History of Flight
On March 10, 2018, at 1646 central standard time, a Czech Sport Aircraft Piper Sport airplane, registration N422PS, impacted terrain at Addison Airport (ADS), Addison, Texas. The flight instructor and one passenger sustained serious injuries, and the airplane sustained substantial damage. The airplane was registered to a private individual and operated by Excite Aircraft, Inc., doing business as US Sport Aircraft, under Title 14 Code of Federal Regulations Part 91 as an introductory/instructional flight. Day visual meteorological conditions prevailed, and no flight plan had been filed. The local flight departed ADS at 1644.
A review of air traffic control audio recordings revealed that at 1642, the pilot requested takeoff clearance while holding short of runway 15. At 1643, the controller instructed the pilot to line up and wait. At 1644, the controller cleared the airplane for takeoff. At 1646, the pilot stated, "we're having vapor lock, we need to come back and land." No further communications were recorded.
Witnesses observed the airplane fly over the southeast side of the airport, descend behind hangars, then climb and make a left climbing turn toward the runway before descending again and impacting terrain on the east side of the runway 33 threshold. Recorded flight track data showed the airplane departed runway 15, made a climbing right turn to the southeast, then continued circling left toward the runway before the data ended near the accident site. Airport surveillance video at 1646 showed the airplane flying southeast above the end of runway 15, descending in a wings-level attitude, then making a sharp left bank turn and impacting the ground.
Aircraft Information
According to the operator's staff instructor meeting agenda from December 28, 2017, the company mechanic discussed fuel blends and vapor lock formation. The agenda stated that if the temperature is over 70°F in the winter, a 50/50 mixture of 100LL avgas and automotive gasoline (mogas) should be used. It also noted that the airplane fuel pump would not fix vapor lock. The operator reported that before the flight, the airplane was serviced with 93-octane mogas stored in their fuel bowser, which was quarantined after the accident. The engine manual noted fuel flow at takeoff performance is 7.1 gph.
Wreckage and Impact Information
The airplane impacted the taxiway and came to rest upright in the grass between taxiway A and the runway, in a left-wing and nose-low attitude, skidding upright. The left wing was broken up and aft; the forward fuselage at the instrument panel was broken down. The airplane was recovered to the operator's hangar and later relocated to a secure storage facility for postaccident examination.
The mechanical fuel pump was actuated by hand and produced a yellowish liquid resembling mogas. A sample tested negative for water and contained no contaminants. No preimpact mechanical malfunctions or anomalies were found that would have precluded normal operation. The fuel return line was installed in a slightly different location than shown in the maintenance manual, but Czech Sport Aircraft stated the configuration was appropriate and did not increase vapor lock risk.
The Dynon engine monitoring system recorded data. During engine start, taxi, runup, and holding short, the engine ran for about 18 minutes. During runup, fuel flow reached about 6 gph and fuel pressure 6–7 psi at 4,000 rpm. During initial climb, fuel flow peaked at 9 gph and fuel pressure decreased to 3.3 psi at 5,025 rpm. Subsequently, rpm and fuel flow fluctuated, with data ending about 15 seconds later. Historical data for the last 10 flights (March 2–10, 2018) showed fuel flow consistently between 7.25 and 8.3 gph during takeoff, with highest flows during afternoon flights when outside air temperature increased to about 75°F. The accident flight showed fuel flow of 8.937 to 9.0 gph in a 10-second span.
Additional Information
According to Electronics International Inc., when air enters a rotor-style flow transducer, the rotor spins at air velocity, representing vapor lock as spikes in fuel flow. Air pulses can cause the rotor to spin back and forth, resulting in jumping fuel flow readings.
Rotax Engines recommended mogas over 100LL avgas to avoid lead deposits and valve seat stress. The airplane's pilot operating handbook (POH) echoed this, with avgas recommended only if vapor lock issues arise or other fuel is unavailable. Rotax and Czech Sport Aircraft recommended mogas meeting ASTM D4814, with minimum RON 95 and AKI 91. The Rotax manual noted, "Use only fuel suitable for the respective climatic zone" and "Risk of vapor formation if using winter fuel for summer operation."
On September 11, 2017, Rotax-Owner.com published an Information Safety Notice highlighting that a fuel return line is mandatory to bleed off vapors and prevent vapor lock. On August 30, 2018, Czech Sport Aircraft issued Safety Alert SA-SC-011, updating the POH with a warning to use fuel formulated for the specific climate zone and not to use winter mogas blends in warmer temperatures.
Tests and Research
Due to fuel system impact damage, insufficient fuel was available for testing. Fuel from the operator's bowser was recovered and tested at a private laboratory, showing a research octane number (RON) of 98.3 and vapor pressure of 12.79 psi. Winter-grade fuel can be as high as 13 psi; summer-grade must be below 9 psi, indicating the tested fuel was winter grade.