Piper Mojave

Piper’s emphasis on the pressurisation of these new types underlines their confidence in the business aircraft market. All three types are aimed squarely at getting executives to their destination in the smoothest, quietest and most efficient manner. The pressurisation gives these aircraft flight planning flexibility and ensures that they operate at higher true airspeeds for the same power settings used at the lower levels.

The Mojave (pronounced MO-HA-VEE) is Piper’s new entrant into the cabin-class twin market. The company had a brief sojourn into this area some years back with the PA31P Pressurised Navajo, discontinued in 1977. Even though there is some similarity between the two aircraft physically, underneath they are really quite different, one of the most notable differences being the dropping of the troublesome Avco Lycoming TIGO-541-E1A in favour of the new, intercooled TIO-540-V2AD.

The easiest way to describe the Mojave is to say that it is a Cheyenne I with piston engines. The uninitiated will find it difficult to differentiate between the two from a distance. Most people, however, will probably draw similarities to the Pressurised Navajo, and whilst they share the same fuselage the Mojave has a new wing that is some four feet longer than the older aircraft and an empennage directly from the Chieftain. The nacelles are also a totally new design, being much cleaner and incorporating much less aerodynamic drag than the P. Navajo. The combination of the higher aspect-ratio wing, lower drag Chieftain empennage and clean nacelles make the Mojave a much cleaner and aerodynamically more efficient aircraft than the P. Navajo.

Apart from the subtle differences mentioned, most Navajo pilots will feel completely at home when they see the Mojave.

The interior also reflects the Piper lineage. The cabin, beautifully done in brown velours, is a direct copy of the Cheyenne. In typical Piper style the windows are large, giving a light airy feel in a comfortable, if not small, cabin. The test aircraft had a club seating arrangement with one extra forward-facing seat at the right-hand rear of the cabin, and two fold-out tables between the facing seats. One enters the cabin via an air stair door which together with the 114 cm x 117 cm cargo door gives excellent access to the cabin for those outsize cargos. Baggage space certainly has not been forgotten with 44.5 cubic feet in the rear cabin and nose locker and 9 cubic feet in each nacelle. This would allow for up to 680 pounds of baggage.

The individual, as usual, can choose his own interior layout from a vast selection of materials and stamp his or her own personality on the Mojave.

The flight deck, as might be expected of an aircraft in the Mojave’s class, was very well equipped, from dual comm and nav equipment to anintegrated Flight Director Auto Pilot system. The test aircraft even incorporated the new Litton LTN 3000 VLF-Omega Navigation system, although this unit was not on its best behaviour on the day.

The main systems and switches are laid out just as they are on the Cheyenne, making it very easy for pilots who fly both types or intend to upgrade to the Cheyenne. Indeed, the flight deck layout is very practical and quite roomy. Once again, the installed equipment is only limited by either your imagination or your chequebook.

The heart of the Mojave, or the one outstanding feature that should make the aircraft a success is the engine. Piper have stuck with the reliable and proven 350 hp Avco Lycoming T10-540 series that also powers the Chieftain and Aerostar 700; however, in the case of the Mojave and Aerostar this powerplant is ‘intercooled’. Actually, there is nothing new about the concept of inter cooling; many of the old supercharged engines had intercoolers, however, up until now no major general aviation manufacturer has bothered to incorporate this feature.

Basically what happens is this: with a standard turbocharger installation, exhaust gases are used to drive a turbine which turn is connected to a compressor. The compressor controlled by a wastegate allows high-pressure air to enter the engine’s induction system, the advantage being that sea-level air pressure can be maintained in the engine’s induction system to 17,000 feet or higher depending on the amount of boost the turbocharger is programmed to supply. Aircraft installations normally utilise turbocharging to maintain pressure at altitude and operate at their peak at high altitude, whereas automotive installations increase the pressure at sea level giving the engine a higher output at sea level. Pumping hot air into the induction system has several disadvantages. Manifold pressures are higher than need be and this ir, turn creates more mechanical stress.

To illustrate the amount of heat created by a turbocharger we note that a compressor operating at a 2:1 compression ratio will raise sea level standard air by 102°C. In conventional turbocharged installations, this air is cooled by adding fuel. Obviously, this creates a richer-than-ideal mixture. The air, being hot, has a lower density and to maintain sea-level pressure higher manifold pressures are used. Now by inter cooling, the air from the compressor passes through a ram air heat exchanger. This cools the compressed air by up to 55°C. This air is then mixed with fuel which further reduces the temperature by another 55°C before entering the induction system. This cool air has far greater density, thereby reducing manifold pressures and allowing the engine to run much cooler. The advantage translates to a bottom-line gain of an engine with a TBO of 2000 hours, which is very good for a relatively powerful piston power plant.

The Mojave flight test was conducted on one of those rare ISA days. Ansett General Aviation’s demonstration pilot, Colin Adams, 700 lbs of avgas and I brought the aircraft’s take-off weight to 6400 lbs or 845 lbs under gross weight. It was not unreasonable to expect a sparkling performance and we were not disappointed.

Starting procedures and ground handling are as positive and straightforward as all the Navajos. Even the run-up procedures are similar, props at 1500 rpm, and magnetos at 2000 rpm. We received our airways clearance and were cleared straight to FL200, on receipt of ‘clear for take-off’ we rolled about 500 metres down Bankstown’s runway 29R to get airborne. Like all Navajos, the aircraft is quite sensitive in the pitching plane yet requires a positive rotation. Typical of the marque, the control column appears to have no end as you pull back.

The Mojave accelerated quickly to 120 kts and we established a very respectable 1800 fpm rate of climb. Not bad for a piston twin even if we were light. Colin said you can usually plan to be at FL200 in 20 minutes. Thanks to our low weight we took about 15 minutes. Hand-flying the aircraft during the climb was a delight, however, we took time out to try the autopilot/flight-director system. This unit worked faultlessly and levelled the aircraft off very smoothly at FL200. We also tried to verify our position with the new Litton LTN 3000 VLF-Omega, but the unit chose not to cooperate. We have since been informed that there was a minor fault in the system. Pity, I would very much like to have seen the unit in action as it shows a great deal of potential at a very realistic price.

With an MAP setting of 29 inches and RPM set at 2300 the Mojave cruised very happily at 210 KTAS, which corresponds with the flight manual. I decided to take a walk down the back during the cruise and found that the passengers could enjoy a comfortable conversation without screaming at one another. Noise levels are quite acceptable, and the seats are very comfortable indeed. On returning to the flight deck we decided to return to BK. Being only some 40 nautical miles out we had to execute several orbits during our descent to give the cabin time to descend and ensure the cylinder head temperatures up. With a 75% power cruise speed of 234 kts and IFR range of 1135 nautical miles the Mojave is a capable aircraft. Couple this performance with a 5.0 psi pressure differential that can provide a 9100-foot cabin at 25,000 feet and this aircraft should provide any corporate owner the flexibility he requires without going to the extra initial and ongoing costs of a turbine.

AGA’s demonstrator as tested was priced at $830,000. That is a lot of money, but then new turboprops are at least double that amount and don’t really offer all that much more.