Pipers PA-31 TwinsWritten by Daryl Murphy
In October of 1958, Piper proposed a twin-engine version of the successful Comanche single. This was not the PA-30 Twin Comanche, the planning of which had begun two years earlier but development and production of which would be several years hence (thus the skip in numeric order).
Piper initially planned for the model to be developed in California by Bill Lear, and would furnish a PA-24 Comanche airframe and two 200 hp IO-360 engines. Whether this actually ever happened is not recorded, but in 1962 the PA-30 project was given to San Antonio designer Ed Swearingen.
In the case of the PA-31, W.T. Piper dictated that, among other things, he wanted an airplane on which the tailcone did not begin its taper until it was aft of the cabin.
Not much is known about what went on until 1964, when Piper engineers at Lock Haven began development of a new cabin-class twin. It was initially named Inca and then changed to Navajo. The six-to-eight-place aircraft was powered by two Lycoming TIO-540-A engines of 310 hp each, and flew on Sept, 30, 1964.
At a gross weight of 6,500 pounds, the PA-31-310 had a top speed of 228 knots and a 75 percent cruise of 217 knots. Piper also made the Navajo available with normally-aspirated 300 hp Lycoming IO-540-M1A5 engines. Although this model, dubbed the PA-31-300, was rated at a lower 6,200-pound gross weight, it proved disproportionately slower, with a top speed of 196 knots and cruise of 184 knots at comparable power settings.
Deliveries of both models began in March 1967, and 90 were sold at an average price of $114,000. A new Navajo flight test building was erected to accommodate the traffic, and a new plant to manufacture subassemblies was built 60 miles northwest of Lock Haven at Quehanna. The 310 hp Navajo proved so popular that the 300 hp version was dropped after only 14 were completed.
In 1970 Piper developed the Navajo B. It had new air conditioning and avionics systems and options of a pilot access door, wide utility door and nacelle baggage compartments.
In 1972, the disastrous flooding of the Susquehanna River devastated the company’s facilities at Lock Haven. Inventory and tooling were ruined and the single-engine Comanche line was shut down for good.
Fortunately, Piper had earlier built a facility in Vero Beach, Fla. to manufacture the Cherokee line, and in the wake of the flood, everything was moved to Florida. Navajo production was accomplished there and at nearby Lakeland.
In 1974 the improved Navajo C debuted, along with the Navajo C/R with counter-rotating 325 hp Lycoming L/TIO-540-F2BD powerplants. The higher-power C/R provided a few knots of speed at cruise at the cost of three gallons of fuel per hour. Average delivery prices rose to $187,500 and $227,600, respectively.
More than 1,300 Navajo 300/310s had been delivered when production ceased in 1984, by which time their asking price had grown to in excess of half a million dollars.
The Navajo II
The PA-31-350 started development in 1968 as a commuter aircraft to counter Cessna’s 402 and the Beech 99. It was basically a Navajo with two-foot fuselage extension that could accommodate two more seats, 350 hp Lycoming L/TIO-540-J2BD engines, and an increase in gross weight to 7,000 pounds. Unfortunately, the second prototype and first production airplane were lost in the 1972 flood.
Piper named it the Navajo Chieftain upon its 1973 introduction, and its average price was $142,000. Production was transferred to Lakeland, where #31-7405401 (N74999) was the first airplane off the line in June. In 1980, “Navajo” was dropped from its name, and production ended in October 1984, when the price had risen to $587,000.
The PA-31P and T
Piper had begun exploring the concept of a pressurized cabin as far back as 1962. In 1966 they began work on the PA-31P. It was a six-seat version of the Navajo with 425 hp TIGO-541-E1A6 engines and 7,800-pound gross weight. It could top 240 knots and cruise at 233 knots. Priced at an average of $275,000 when it was introduced as a 1970 model, just over 100 were sold before production ceased in 1977.
In 1980, Piper developed a new pressurized cabin-class twin, the PA-31P-350 Navajo Mojave. This aircraft, with its Cheyenne I fuselage, 350 hp counter-rotating Lycoming L/TIO-540-V2AD engines, the Chieftain’s tail unit and the wings from the PA-31-353, was short-lived. Deliveries of the Mojave—“Navajo” was dropped from the name—began in July 1983 and ended in June of 1984. But the Mojave had given the company valuable experience in pressurization, which led to the Cheyenne series and eventually to the single-engine Malibu and Mirage.
To prove the concept of turboprop engines on a Navajo airframe, Piper pulled #31-1 and sent it to Swearingen Aircraft in October 1966, where 500 shp Pratt & Whitney PT6A-20 turbine engines and Hartzell three-blade props were installed. It first flew in April 1967 (and last flew in 1969, after which it was donated to the local fire department).
Piper installed two 620 shp PT6A-28 turbines on the Pressurized Navajo airframe and added wingtip fuel tanks in 1969, also modifying the flight control system to accommodate the increased power. The model was named the PA-31T Cheyenne and was certified in May 1972, but production at Lock Haven was delayed because of the flood. The first production airplane rolled out in May 1973 and deliveries began the next spring.
In 1978, the lowest-priced turboprop on the market was introduced—the PA-31T1 Cheyenne I (at which time the original Cheyenne was renamed the Cheyenne II), base-priced at about $500,000. It reverted to 500 shp PT6s and deliveries began in mid-1978.
The next year, the PA-31T2 was developed, a Cheyenne II with 24-inch fuselage stretch. Other variations followed, but softening General Aviation markets and rising costs forced manufacturers to make drastic cuts—and, in some cases, discontinue models in the mid-1980s.
All told, however, Piper Aircraft produced nearly 3,400 PA-31 twins and several hundred turbine models based on the basic PA-31 design.
Piper Aircraft: The development and history of Piper designs. Roger W. Peperell. Air Britain, 1996.