A Twin-Engine Amphibian with Italian Flair
The available examples of twin-engine general aviation amphibian airplanes make a very short list, and their marketing success has been quite limited, for various reasons. To build a rugged airframe for water use, light enough to be powered by relatively small piston engines, at a cost within reach of primarily recreational users, is a difficult set of tasks. And then there are the hurdles of training and insuring low-experience pilots…
Even so, there’s a universal appeal engendered by the freedom and utility of aerial yachting. To be able to land friends and family on both water and runways, visiting remote beaches and urban airports alike, opens up worlds beyond that of normal flying.
Captain John Mohr, who pilots Boeing 767s for Delta Airlines, is uniquely qualified to talk about this distinctive aspect of aviation, and his unique airplane, a Piaggio P.136 Royal Gull. The beautiful Gulls were designed and built in the 1950s (the prototype flew in 1949, using 215-hp Franklin engines similar to the Republic SeaBee’s), a creation of the Piaggio company in Genoa, Italy. It was obvious that more horsepower was needed than that offered by the Franklins, so Piaggio turned to the most powerful small engines available at the time, the geared GO-series from Lycoming. Used in Aero Commanders and Twin Bonanzas of the era, the Piaggio’s engines were given flanged crankshafts in place of the normal splined shafts, in order to mount reversing propellers.
The Gull is a pusher design, with the wing built in an arched seagull shape to keep the engines high during water operations. This places the cabin well forward of engine and prop noise, and it also allows for easy nose-in beaching, made easier by a swing-open windshield as well as two large side doors. It’s an impressively large airplane when sitting on an airport ramp, with a height of over 12 feet and an arched wingspan of 44 feet. Given its conventional (tailwheel) landing gear, the engines are well above head height. On the water, the aircraft takes on a more level attitude, drawing a mere 24 inches of draft at dockside.
Captain Mohr is what you might call a veteran water pilot. His grandfather was an early-day barnstormer, who later operated a flying service at Crane Lake in northern Minnesota. John Mohr learned to fly while sitting on his father’s lap in a float-equipped Piper J-3 Cub, starting at about age 5; he unofficially soloed in that Cub at age 14. The airplane was bought new by his grandfather in 1946, and it’s still in the family.
John Mohr worked through his growing up years at all manner of aviation jobs, flying Cessna floatplanes in his family’s operation, working for a charter operator flying Piper Aztecs and doing a stint in Twin Beech freighters. He also flew airshows for over 25 years in a stock Stearman PT-17, retiring after the 2013 season. He joined North Central Airlines in 1977, first flying the venerable “Blue Goose” Convair 580 turboprops, and he has moved through various acquisitions of the airline to his present seat in the company’s 757/767 international operations.
He bought his Royal Gull in 1993 and has flown it for about 1,500 of its 3,500 total hours. Among its significant mods are the change to reversing Hartzell propellers and the installation of 295-hp GO-480-G2D6 engines, as used on the Beech Twin Bonanza D50. Mohr has the ability to install a 70-gallon ferry tank for long flights, giving him 7.5 hours of endurance. In the year 2000, he flew his Gull across the North Atlantic to Southampton, England for a special Y2K celebration, and in 2015 he flew another Gull, a P.136-L2 with 340-hp supercharged GSO-480 engines, over to Italy, visiting the aircraft’s birthplace. Normally based in the Twin Cities, he’s had his airplane to the Bahamas and other Caribbean destinations, he’s gone as far south as El Salvador in Central America, and he based it in Alaska one summer, using it for fishing and exploring during airline layovers.
The American Connection
In all, some 65 P.136 Gulls were built by the Piaggio company, about half of which were assembled and sold in North America by Milwaukee company Kearney & Trecker, an industrial machine tool supplier. President Francis Trecker was on a trip to Italy in the early 1950s and was impressed with the Piaggio airplane’s versatility, so he acquired the North American rights for his company. The airframes were shipped by container to Milwaukee and the engines, propellers and avionics were installed by Trecker Aircraft Company, which marketed it as the Trecker Royal Gull. The name “Royal Gull” was actually applied by Kearney & Trecker, which initially formed a subsidiary, Royal Aircraft Corporation, that later became Trecker Aircraft.
The Piaggio P.136 is one of the few foreign airplanes to receive its own FAA (actually CAA Part 10.30) utility-category type certificate, #A-813, originally issued August 15, 1955, rather than being certificated under reciprocal agreement. The P.136-L had 260-hp GO-435-C2B engines, the –L1 was fitted with 270-hp GO-480-B1B engines and the –L2, certificated in normal category on March 7, 1957, carried the 340-hp GSO-A1A6 supercharged engines.
Because of their size and weight, the first Gulls didn’t have outstanding single-engine performance. The factory specs for the – L1 showed a single-engine climb rate of 216 fpm at sea level and a 4,100-foot single-engine service ceiling, with a 5,000-foot absolute ceiling. Nevertheless, with two engines running it could take off in less than 1,000 foot of runway, and Mohr says it uses about the same water distance as ground roll. Lightly loaded, he can match a Super Cub in takeoff performance. The supercharged – L2 version improved SE performance considerably, despite an increase in gross weight to 6,614 pounds on land, 6,393 pounds on water.
The engines turn 3,400 rpm for takeoff, but are immediately reduced to 3,000 rpm for the maximum continuous power rating. Cruising at 2,750 rpm and 21 inches manifold pressure, Mohr achieves 140 knots, burning about 34 gallons per hour. Yes, other similarly-powered airplanes are faster, but they are only good for one water landing.
The Gull’s over-the-wing engine cowlings prop open for easy maintenance access (with ladders or from the wingwalks). As with any pusher design, care should be taken during preflight to make sure no loose objects are poised to travel back to the propellers. There are no cowl flaps, no fuel tank management is required, and the gear’s hydraulic system is powered by an electric power pack under the three-place rear seat, backed up by a hand pump. The flaps and tailwheel lock are also hydraulically-actuated; the flaps can be extended to any setting up to 45-degrees.
Fuel is contained in a pair of 95-gallon aluminum tanks in the fuselage, each one feeding its respective engine. The maingear swings up into open wheel wells under the wingroots, the tires remaining partially exposed while the struts are covered by fixed doors. Main tire size is 8.50 x 10. The 5.00 x 5 tailwheel also retracts, and it is not steerable, so brakes are needed for ground maneuvering. It does lock into a straight position for takeoff and landing. The water rudder below the tail extends and retracts, activated by a handle on the overhead console, allowing precise water maneuvering without differential power. The fuselage is divided into seven water-
Permission To Come Aboard
The cabin, reached from either side by climbing up to the waist-high door sill, seats five with two individual seats forward and three seated aft on a wide bench. Up to 120 pounds of baggage can go behind the rear seats, and a separate aft baggage compartment in the tail, adjacent to the fuel tanks, can take 300 pounds. In addition, there’s a line and anchor locker in the nose, reachable by swinging open the right half of the windshield. A 20,000-BTU Southwind cabin heater is in the aft fuselage. Electrical controls are located overhead, along with the starting controls and rudder trim crank. Unlike most twin amphibs, the power levers are on a center pedestal, rather than hanging down from the overhead. The propeller reversing levers are isolated on the left side of the panel. A controls lock pins the right yoke and rudder pedals when moored. The Lycoming GO engines are fitted with an altitude compensating pressure carburetor, so the mixture knobs are merely used for idle-cutoff.
Clearing propellers for starting requires a bit of a stretch to see far aft, but the start-up is otherwise conventional. Taxiing visibility is nearly unobstructed by the nose in landplane configuration, due to the shallow deck angle. Runup requires 2,600 rpm, a product of the 77:120 gear ratio; there are no abnormal items to be checked. Mohr says 2,000 feet of normal water surface is adequate for takeoff with four aboard. He reports minimal prop erosion during water takeoffs, thanks to the high-mounted engines. Vmc is 90 mph indicated, and the best single-engine climb speed is 95. The propellers turn at a mere 2,180 rpm when the engines are at their full 3,400 takeoff rpm. A 120-mph climb speed is normally used; after takeoff, throttles are left full open and the rpm is pulled back to 3,000 for max continuous power. Once gear and flaps are up, 25 inches m.p. with 2,850 rpm is a normal climb power. Cruise rpm is an economical 2,600, or 2,750 for max cruise on 21 or 22 inches.
Half flaps are commonly used for minimum run takeoffs, and they can be lowered at speeds up to 150 mph during approach. The L1 version of the Gull had a maximum gear extension speed of 126 mph, which was also its limit for additional flap extension, but the L2 allowed the gear to be extended at 161 mph IAS, with full flaps permitted at 129. Stall speeds are reported to be about 75 mph with flaps up, and around 68 mph in dirty configuration. As with any amphibian, knowing gear position for water landing is critical, so there are mechanical gear indicators as well as lights.
Yes, there was a landplane version of the Gull, the Piaggio model P.166, which was first flown in late 1957. It was equipped with a nosewheel, with the maingear moved farther aft, and it had a prominent dorsal fin. The P.166 cabin initially seated six, later enlarged to hold 10 or 12. Instead of the P.136’s wing floats, wing tip fuel tanks were added to increase fuel capacity for the 340-hp GSO-480 engines, and eventually Lycoming LTP 101 turboprop engines were fitted.