July 2005- One time-tested way to create a light twin is to take a single, remove the engine, and replace it with two engines on the wings. Sometimes the engines are smaller than those on the single—as in Piper’s Twin Comanche. Sometimes the engines are as powerful as that on the single—as in the Beech Baron.
Piper took the first approach with the PA-34 Seneca: it’s basically a Saratoga airframe with the 300 hp single engine replaced by two 200 (later 220) hp wing engines. The result is one of the longest-running twins in General Aviation.
The original Seneca was introduced in 1972, and you can still buy a brand-new Seneca V from Piper today. The original PA-34-200 Seneca had some problems, notably a low single-engine service ceiling.
It was built with two normally aspirated, four-cylinder 200 hp Lycoming IO-360 engines. Together, they provided 400 hp at sea level, 25 percent more power than was available from the 300 hp engine used in the Saratoga—but with one engine out, the Seneca was running on only 200 hp… or less, at altitude. The result was an extremely low single-engine ceiling, about 5,000 feet in the original aircraft.
Piper also went to great lengths to make the Seneca spin-resistant, using counter-rotating props to eliminate “critical engine” issues, and providing an aileron-rudder interconnect system to minimize the risk of crossed controls and unintentional spins. Unfortunately, the result was an airplane that didn’t behave well in turbulence.
In 1975, Piper dealt with these problems by introducing a new model with turbocharged, six-cylinder 200 hp Continental TSIO-360 engines. This raised the single-engine ceiling to over 13,000 feet and provided a smoother ride with less vibration. Piper also removed the aileron-rudder interconnect and rebalanced the controls.
The 1981 Seneca III upgraded the engine power to 220 hp (for 10 minutes—then you have to reduce to 200 hp, where it remains today), reinforced the structure, and switched from manual to electric flaps. Minor changes to the cowl and engine resulted in the Seneca IV and V respectively.
I have a soft spot for the Seneca—it’s the first (and only) twin in which I’ve been able to log time. I had the privilege of flying a Seneca III and V for a total of about 10 hours during a visit to Texas a couple of years ago.
Piper intended the type as a step-up airplane for complex single-engine pilots, and I think they did a good job; for a pilot with time in an Arrow or Saratoga, it’s a relatively simple transition. Differences, though, begin with preflight planning.
The normal procedures section of the Seneca III POH instructs you to calculate time, distance and fuel to both climb and descend, and only then figure the cruise segment—which makes sense for a turbocharged airplane that frequently cruises in the flight levels.
Also worth noting is the optional combustion heater (helpful at high altitudes—no engine in the nose means it can get bloody cold above the freeze level!), which burns about .5 gallons of Avgas per hour.
The accelerate-stop distance is also important, as you need to know if the runway is long enough to let you change your mind in case of engine failure before you break ground.
Preflight calculations also cover weight and balance—and the Seneca is definitely not an airplane you can fill the tanks and seats in: Basic empty weight for a Seneca III is 3,212 pounds, max ramp weight is 4,773 pounds, and takeoff weight is 4,750 pounds (figuring that you’ll burn 23 gallons on start/taxi & hold). That yields a respectable 1,400-plus pound useful load… but 90 gallon standard tanks means 545 pounds of that is fuel (more with the optional long-range 128 gallon tanks). That makes the full fuel useful load only about 900 pounds, in a six- or seven-seat airplane!
The preflight inspection is very similar to that for a complex single, but you have to check oil levels in both engines. Depending on the model, Senecas can have up to six fuel tanks, so there are quite a few fuel test ports to check.
The Seneca offers passenger accommodation similar to a Saratoga (including club seating in later models) and plenty of baggage space in forward and rear compartments (100 pounds each).
Pilots, however, will find a much more complex instrument panel. Since this is a twin, there are twice as many engine gauges as on a single (in some cases, single gauges with two needles), and most Senecas have a three-axis autopilot and flight director (FD) to complement an HSI.
This can be a big change from the more familiar attitude indicator and directional gyro, but once you learn to use them, the FD and HSI make life a lot easier, particularly when flying an instrument approach. That leaves the pilot time to pay attention to the engines, which is critical in the Seneca.
The TSIO-360 engines used in Senecas come from the factory with a fixed turbocharger waste gate, which is mechanically simple but requires the pilot to pay attention in order to avoid overboosting the engines.
This is apparent on takeoff—you don’t firewall the throttles in a Seneca. If you do, you’ll blow the overboost pop-off valves when you exceed 42 inches MP, and wind up with normally aspirated engines (and an expensive repair, though cheaper than replacing an entire engine).
The takeoff procedure in a Seneca is to taxi onto the numbers, stand on the brakes, and slowly wiggle both throttles up while watching the manifold pressure gauges. You stop at about 30 inches, and as the turbos spin up you’ll indicate about 35 inches—maximum takeoff power is 40 inches MP at 2,800 rpm.
Once they’re up to power, release the brakes and start to roll. Rotation speed is about 80 knots, with liftoff at 85—immediately lower the nose to blue line (single-engine best rate), or 92 knots. Raise gear when you run out of runway, with a positive rate showing.
Once the gear is up you can lower the nose to a cruise-climb configuration at 75 percent power (33 inches MP at 2,600 rpm), which helps to keep cylinder head temperatures down. (The POH recommends cylinder head temps below 420 degrees and oil temp below 200 degrees F in cruise—controlled by power reduction, cowl flaps and/or mixture.)
The prop levers are handled like the throttles—but after you get everything else set for cruise you may wind up playing with the prop controls a bit to eliminate the “wah-wah” sound that results from the props being slightly out of sync. Optional synchrophaser controls on some Senecas are supposed to do this automatically, but they don’t always work.
Minimum single-engine control airspeed in the Seneca is just 66 knots, but Piper recommends 85 knots as single-engine demonstration speed (Vsse). Instructions for loss of an engine in the POH tell you to identify the lost engine by noting yaw with controls coordinated, confirm by pulling the throttle on that side and then compensate with rudder—but expect the wing on the dead side to be up to five degrees low.
Piper’s instructions for engine-failure on takeoff are pretty explicit: If airspeed is less than Vsse (85 knots) and you haven’t broken ground, then you’re instructed to close the throttles, stand on the brakes “and steer to avoid obstacles” rather than attempt a single-engine takeoff.
If airborne but at an airspeed below Vsse, close both throttles and land straight ahead (if you’re high enough in climb you can think about opening the throttle back up on the good engine after picking up speed in the resulting dive).
The POH strongly discourages single-engine go-arounds (after looking at the single-engine climb performance table I can see why; the best you can manage is 600 fpm, and that’s at sea level on a cold day!)
Balked landing climb performance (assuming 40 degrees flaps and gear down) is in the 600-800 fpm range on both engines (which probably explains why the single-engine landing procedure is restricted to 25 degrees of flaps).
Senecas are available with known icing equipment (heated windscreen, prop de-ice and wing boots), but the Seneca III POH includes a three-page insert warning that even though the airplane is certificated for flight in known ice, you can’t just fool around with severe ice.
On encounter with a possible severe icing situation (freezing rain, etc.) you’re instructed to “request priority handling from ATC” (I’d declare an emergency); descend; keep airspeed above stall; take firm grip on the wheel and disconnect the autopilot. The POH also recommends avoiding moderate ice at FL100 or higher.
Typical cruise in a Seneca at 65 percent power gives you 165 to190 knots depending on altitude (better high) and 128-gallon extended-range tanks (122 gallons usable) gives you a range of more than 800 nm with 45-minute reserve.
Maximum speed is over 200 knots, so you can begin descents by lowering the nose and allowing speed to come up—but it’s essential to reduce power slowly to avoid shock cooling. Gear and the first notch of flaps can be lowered at 140 knots and will slow the airplane by about 20 knots, setting you up for final approach.
The Seneca is a stable airplane for both instrument and visual operations, and it’s easy to land. With total landing distance of 2,180 feet over a 50-foot obstacle, and takeoff distance of just 1,700 feet, it offers pretty good short-field performance for a twin.
Senecas can be among the most economical turbocharged twins to operate. A medical specialty practice that covers most of the southern half of Texas operates three of them, which allows one doctor, with his support staff (two technicians) and equipment, to cover multiple clinics in that area. At any one time, only two of the airplanes are in use (which allows for preventive maintenance).
Two years ago, annual cost for the flight department—including all three airplanes, three pilots (salaries included) fuel and maintenance, came to under half a million dollars for some 1,300 flying hours. They figured the hourly cost to operate their Senecas at just $150, which is competitive with many singles. By comparison, a nearby hospital flight department was using Cessna 421 pressurized twins for much the same job—at about $500 per hour!
Proper operating practices, though, are essential to keep costs down. Pilots working for the medical practice in question use 75 percent power for climb, 65 percent power in cruise, and monitor their engine instruments (particularly cylinder head temperature) closely. The airplanes are flown over 400 hours a year and get regular maintenance.
Using those procedures, they’ve been able to reach TBO on all engines, consistently, for many years—exchanging them for factory remans. Their business manager told me that the Seneca actually costs them less to operate than a Saratoga, mainly because of maintenance on the air conditioning system.
The Seneca remains in production today, with the latest Seneca V offering advanced features including dual alternators, dual Garmin GNS-530/430 GPS Nav/Coms, leather seats, S-TEC 55x dual-axis autopilot, and extended-range tanks as standard equipment. Options include weather radar, KMD-850 multifunction display, full copilot instrument group, Megitt EFD, de-ice equipment, three-blade props, synchrophaser, air conditioning and built-in oxygen among other things.
Prices for used Seneca IIIs can be below $200,000, while a new Seneca V will set you back well over half a million (with a two-year warranty), depending on options.
A wide range of mods are available for Senecas, including wing tips with landing lights (standard on new Seneca Vs), gap seals and fairings and modified cowls. Merlyn products sells an automatic wastegate kit that the company claims will increase performance while simplifying operation and eliminating the risk of inadvertently overboosting the engines.
Want to find out what it’s like to fly a Seneca without buying (or spending a couple of hundred bucks to rent) one? FSD International offers a $29.95 Seneca V add-in to Microsoft Flight Simulator 2004. It includes an accurate panel with dual Garmins, proper aerodynamic modeling including the counter-rotating props (and resulting reverse torque effect in single-engine operation depending on which engine you shut down), and coupled autopilot.
John D. Ruley is a freelance writer, instrument-rated private pilot, and volunteer pilot for LIGA International (ligainternational.org). Send questions or comments to editor@www.piperflyer.com.
Resources
Piper Aircraft
piper.com
RMD Aircraft Lighting Inc.
rmdaircraft.com
Knots 2 U Ltd. (gap seals, fairings, wingtips with landing lights)
knots2u.com
LoPresti Speed Merchants (spats, splitters, seals, wingtips with landing lights, cowls)
speedmods.com
Merlyn Products Inc. (automatic turbocharger wastegate)
merlynproducts.com
FSD International (Seneca V add-on to Microsoft Flight Simulator 2004
fsd-international.com/projects/seneca.htm
