sought to alter the plane's single-engine behavior at low airspeeds (such as takeoff and landing). The resulting PA-39 Twin Comanche C/R was a modified version with counter-rotating engines (to eliminate the "critical engine" which, when failed, created the greatest risk of loss-of-control). The PA-39 replaced the PA-30 in the early 1970s. At least one comparative study of U.S. crash rates for the PA-30 and PA-39 indicated that crash rates for the counter-rotating PA-39 were only one-third of those for the conventional PA-30. Piper subsequently expanded the counter-rotating engine development to its other light twins. As of November 3, 2019, 80 PA-39's remain on the Federal Aviation Administration (FAA) registry. Variants Three versions of the PA30 were produced: the original, a B model, and a C model. The B and C models can carry six passengers and can be identified by the additional side windows. Factory turbocharged engines became available with the B model. Turbo models can be identified by the presence of vents on the nacelle sides. These use Rajay turbochargers with manual wastegates operated by twin knobs under the throttle quadrant. They are properly termed "turbonormalized" since the compression ratio and maximum manifold pressure remained unchanged. Pilots are required to use care to avoid overboosting at low altitudes, since no overboost popoff valves are used. Since there is no accompanying power increase, the B/C models simply give owners a fuel vs. passenger tradeoff. The two rearmost seats, occupying the baggage compartment, are quite small. PA-30 Twin Comanche Four-seat twin-engined cabin aircraft powered by two 160-hp (119-kW) Lycoming IO-320-B1A engines in wing-mounted nacelles. The aircraft was available in five levels of equipment fit: Standard. Custom - additional instruments Executive - additional instruments and radio equipment Sportsman - Palm Beach interior and exterior and AltiMaticII autopilot Professional - full IFR panel PA-30 Twin Comanche B PA-30 with a third cabin window on each side and an optional fifth or sixth seat PA-30 Turbo Twin Comanche Turbonormalized version of the PA-30B Twin Comanche, powered by two Rajay turbonormalized IO-320-C1A piston engines PA-30 Twin Comanche C Twin Comanche B with new instrument panel, switches, and other minor variations. Optional turbonormalized version. PA-30-200 Experimental variant with two 200hp Lycoming engines, one built PA-30-290 Experimental variant with two 290hp Lycoming IO-540-G engines, one converted from PA-30 prototype PA-30 Twin Comanche D Original designation for the PA-39. PA-39 Twin Comanche C/R PA-30 with counter-rotating 160hp Lycoming