Pilot Report: Flying a Walter-powered Beech King Air C90

 - August 8, 2008, 9:52 AM

I recently had the chance to fly one of the King Air C90s re-engined with two Walter M601E-11s, which are 751-shp engines flat rated to 550 shp for this installation. The airplane we were flying was N800RP, a 1974 King Air C90, S/N LJ-628. Dan Sigl, owner of Seagull Aviation, which is working on a conversion package for King Air 90s and 100s, agreed to bring the aircraft to my home base, Solberg Airport in New Jersey. Sigl is associated with the STC holder, Performance Conversions, Incline Village, Nev. Another company, LoneStar Propjets, Waco, Texas, has a separate STC.

The first thing you notice about the modified twin turboprop when you start your preflight is the big five-blade Avia props and the square exhaust stacks. The preflight procedure for the Walter conversion is the same used on any other King Air. On the day of our flight we topped off the tanks so that with four people and 125 pounds of baggage the aircraft would be at mtow and within c.g. limits for takeoff. The OAT at the airport’s 200-foot elevation was 23 degrees C, and there were scattered clouds at 3,000 to 4,000 feet. Calculated takeoff distance over a 50-foot obstacle was 3,400 feet.

The before-start procedures are standard for both the Walter- and Pratt & Whitney Canada-powered aircraft. The starting procedures for the Walter are changed from the Pratt, however. Walter recommends using an APU for starts, but we did not use one on the day I flew the airplane and the temperatures were well below any limits. Before starting the engine, the igniters are placed in the test position, high and low, and you listen for their operation.

The start sequence is automatic for the Walter. Place the start switch in the on position for two seconds then move it to off and the 20-second start sequence is initiated. Move the condition lever to on and monitor N1 and ITT. The ITT must rise within 12 seconds or the start must be aborted with the condition lever (no-light start), and must remain below 730 degrees C (hot start).

Once the engine start sequence is complete, the pilot sets N1 to 75 percent and turns the generator on when not using the APU. When the ammeter indicates less than 50 percent, repeat the process for the other engine. The operating generator stays on to assist the second engine start.

After completing the after-start checks, we taxied out for departure on Runway 22. The ground handling of the airplane was normal in most respects. The one difference that I noticed is the heavy force required on the power levers when going from flight idle to the Beta range. The before-takeoff checks are essentially the same as for the Pratt-powered King Air with two exceptions: the overspeed governor and secondary low-pitch stop checks are not required with the Walter/Avia installation and the bleed valves are set to off for takeoff.

Up and Away

The takeoff was made with takeoff power set before brake release. The acceleration seemed to be normal for the weight and conditions. It took 23 seconds and about 2,200 feet to achieve a positive rate of climb and retract the gear.

Passing through 1,000 feet, we were established at 112 knots, which is the published Vy for all weights. A check of the stopwatch showed 60 seconds from brake release and 37 seconds from rotation. We were able to climb in VMC from 1,000 feet to 15,000 feet in 9 minutes 3 seconds, for an average climb rate of 1,547 fpm. The Beech C90 flight manual time-to-climb chart shows that the unmodified aircraft would have taken 17 minutes to do so. In our climb to FL250 I saw the VSI indicating 900 fpm between FL220 and FL240, with the airspeed stabilized at 112 knots.

Leveling off at 15,500 feet, I set max cruise power. The high teens is as high as anyone would normally fly a C90 but the Walter engines make flight at the airplane’s 25,000-foot ceiling a reasonable solution to getting over the weather.

At the completion of our evaluation at 25,000 feet, New York Center gave us a clearance for a rapid descent to 9,000 feet. With the power levers at flight idle and the airspeed pegged at the 208-knot Vmo, we were able to come down in excess of 4,000 fpm.

Leveling at 9,000 feet I began a stall series, which was completely benign as one would expect in a King Air. Power-off stalls both clean and dirty were straight ahead, with no tendency to break off into a spin. Power-on stalls resulted in a pronounced nose-up attitude and did require some work with the rudder pedals to keep it going straight ahead. Steep turns using 45 degrees of bank were hands-off once the aircraft was trimmed. Bringing one engine to flight idle, I began a series of turns at 110 knots and the aircraft was quite well behaved, with control inputs well less than full travel.

I was curious about the effect of the five-blade Avia props on Vmca. The standard C90 Vmca of 92 knots did not change with the Walter conversion. With the left engine at flight idle and the right engine at max continuous power, with five degrees of bank into the right engine, the stall warning went off before I ran out of rudder travel, at which point the recovery was made by gently lowering the nose and allowing the aircraft to accelerate.

The traffic pattern and landing are standard for the Walter King Air: fly final at a Vref of 100 knots until reaching 50 feet agl, then reduce the power to idle. There was little to no float in the round-out and the aircraft came to a stop about 2,000 feet from the threshold without using the reversers. I watched Ziad “Z” Nazif, the chief pilot for MPC of Marion, the owner of the aircraft, bring it into Solberg for the flight test and he used the reversers on landing and came to a stop in about 1,500 feet. The flight manual supplement for the Walter conversion calls for a landing distance of 3,400 feet over a 50-foot obstacle at the 9,700-pound mlw at sea level and 25 degrees C.

Of note, I thought that the force required to bring the power levers into reverse on this aircraft was excessive. It would be very difficult to finesse the props on an icy ramp in the winter.

The Walter Engine

Walter is an engine manufacturer that is better known in Europe than in the U.S. The company was founded in 1911 by Josef Walter, with the company producing motorcycles and motor tricycles. Production of licensed BMW inline water-cooled aircraft engines started in 1923, and in 1952 the first M05 jet engine was delivered. The engine powered a Czech version of the MiG-15, which was exported to many countries. Production of the first Walter M601 turboprop engine began in 1975.

The M601-series engine was originally designed for the Let L-410 regional turboprop. A reverse-flow engine, the M601 has two independent sections–the gas generator and the power section. The gas generator and power turbine shafts are arranged in a tandem layout. Air enters the engine in the rear, flows forward through the compressor, combustion chamber and turbines and exits through exhaust nozzles near the front of the engine.

Air enters the compressor in a radial direction via a protecting screen and annular plenum. The air is axially directed in front of the compressor, which consists of two axial stages followed by one centrifugal stage. The combustion chamber is of annular configuration. Part of the primary air enters the flame tube through the perforations in the wall, and the remainder passes through the hollow nozzle guide vanes of the gas generator turbine.

Unlike the PT6 engine, the fuel in the M601E is atomized by a special spray ring rotating with the gas generator shaft. The one-stage gas generator turbine drives the compressor via the compressor shaft. The inter-turbine temperature is measured by nine thermocouples installed in the flow path at the gas generator turbine outlet.

The one-stage power turbine drives the propeller via a two-stage counter-shaft-reduction gearbox. The reduction gearbox embodies an internal torque meter, which provides indication of engine power.

Exhaust gases from the power turbine pass through the annular plenum to the atmosphere via two opposed exhaust nozzles, providing for additional “jet” thrust.

The engine’s fuel system is a low-pressure design with a gear fuel pump. The engine is started by an electric starter/generator and two torch ignitors.

The oil system is a circulatory pressure system with an integral oil tank incorporated into the accessory gearbox. This system provides lubrication for all areas of the engine and oil pressure for the torque meter and propeller pitch control.

The engine is equipped with limiters that prevent it from overheating or over-speeding during start, reverse and in Beta. The limiters indicate the exceedance of permitted values of torque, gas generator RPM, propeller RPM or inter-turbine temperature.

The powerplant is controlled by three sets of levers. The power levers control the power output of the engine and the propeller blade angles in Beta and reverse. The second set of levers controls the propeller speed via the primary prop governor and emergency propeller feathering. The third actuates the fuel shutoff valve and, if an emergency circuit is on, it controls engine power.

Temperature limits are ISA+37 for both the Walter and the Pratt. The Walter, designed for Russian winters, has no minimum temperature limit for start but does require preheat when ambient temperatures are below -20 degrees C. The Pratt has a minimum oil temperature of -40 degrees C for starting.

The Avia V510 that comes with the Walter conversion is a full-feathering, constant-speed, reversing five- blade propeller. It has forged aluminum-alloy blades, is hydraulicly controlled and has a 3,000-hour TBO with no calendar time limits.

Both the Pratt- and Walter-powered King Airs are approved for IFR day and night operations and flight-into-known-icing conditions.

The maximum altitude for the Walter-converted King Air has been reduced to 25,000 feet from the Beech limit of 30,000 feet. This is not significant since the two-engine climb rate on a Pratt-powered King Air is only 50 fpm at FL260 in ISA conditions. The airspeed limits are unchanged on the Walter-powered conversion.

The mtow for the Walter-powered King Air gets a boost from the standard 9,650 pounds to 10,100 pounds, which balances out the 360 pounds that the airplane gains in empty weight with the conversion. The maximum landing weight goes up to 9,700 pounds from 9,168, and the zero-fuel weight for the Walter-powered ship is 9,000 pounds, where there was no ZFW limit on the original.


The M601E-11 has a TBO of five calendar years or 2,000 hours or 2,250 cycles for overhauled engines, and five calendar years or 3,000 hours or 3,300 cycles for new engines. The standard calendar limit may be extended for an additional two years by having a detailed visual inspection performed by an approved facility.

Maintenance training is provided by Walter. The one-week course usually takes place in Prague, Czech Republic, but in special cases it can be held at a customer’s place of business.

Conversion to the Walter engines will cost about $500,000 and takes about 12 to 14 weeks downtime. The cost can be reduced by selling the old engines, and all of the companies performing the conversion will assist in the sale of these.