What to look for in a prebuy?
 

All, Im about to buy a 75 Pressurized T337G, I have a good list to check from people that I know who has flown skymasters... I do have a guy to help assist with a prebuy, but for my own knowledge and understanding Im hoping maybe you guys/gals could help on what to look for and add to my small list to check.

Thanks

Test the pressurization to be certain it can maintain max differential. If there are leaks, they can be expensive to find and fix.

If there is a recording engine monitor like a JPI, download the data and send it Savvy Aviation for analysis. They can really give insight into the health of the engines and individual cylinders.

Check for oil leaks on the rear engine. They are SUPER common.

Check the plane after sitting overnight or even better a few days to see if the gear doors start to droop down. If they do, your gear accumulator is bad. Its located under the floor near the pilot's seat.

Test the gas-fired heater and be certain its been inspected on schedule. Surprising to me how often this gets overlooked, especially in warm climates.

Check to see when alternator drive couplings were last changed. Mine had just been replaced at about 800 hours when I bought my P-model. They typically wear out before TBO. If they fail, they cause serious internal engine damage.

Test flight for fuel flows (are they matched?), throttle positions (are they aligned?), oil pressure/temps, make sure all gauges have correct indications. Check the AP for holding altitude and heading. Check the NAV function on both a VOR/ILS and a GPS fix (if coupled). Check the electrical system by turning off each alternator to be sure the other alternator picks up the load. Check to see if the alternator warning lights are blinking during flight (another very common scenario that my 400-series voltage regulator upgrade is designed to fix - $499).

Check fuel gauges for correct indications throughout the range - they are a known weak point in the Skymaster.

Be sure the powerpack is topped off with hydraulic fluid (its behind the center pedestal cover).

I'm sure I missed some things, but this is a start at least.

Thanks for the info! Im actually looking at buying one of our members plane if the prebuy goes well this Monday.. Its is Barry Sloane (N187) plane.

Here is what I have so far... Anyone, please feel free to add to list of known skymaster problematic areas or issues.


 FUEL LINE LEAKS
-Look for Obvious Fuel Line Leaks

 OIL LEAKS
-REAR ENGINE notoriously known for oil leaks.

 FULL FUEL CHECK
-If you fill the tanks too fast the inner tanks does not fill as quickly as
the outer tank and can cause you to lose up to 12 gals.

 MAG CHECK REPLACED?
-Mags are good for about 300-400 Hrs
-Are they Pressurized Mags?

 FUEL PUMPS - need clarification
-something about setting fuel pump to LOW and if the engine needs
more fuel pressure, they automatically go HIGH?

 ENGINE MOUNTS
-Check engine mounts (especially the rear)

 ALTERNATOR
-Check Alternator drive couplings for slippage. The wear out before
TBO, and can cause Internal Engine Damage.

 VACCUM PUMPS
-Most of the time they are 300 hrs pumps and need serviced.

 LANDING GEAR DOORS
-Check for gear door droop. If they do, the Gear Accumulator is bad
(located near under the pilot seat)
-Manually Cycle the Gear

 LANDING GEAR POWERPACK
-Top off with Hydraulic Fluid (behind center pedestal)


IN FLIGHT

 POWER ON TAKE-OFF
-Make sure the Power Red Lines on take-off

 JANITROL HEATER
-Verify functioning (Just replaced / refurbished)

 FUEL FLOWS
-Check that Fuel Flows Match

 THROTTLE POSITION
-Verify throttle positions are aligned

 GAUGES
-Make sure all gauges have correct readings.
-FUEL GAUGES ( Right Gauge was just replaced)
-OIL PRESSURE
-ENGINE TEMPERATURE

 ALTERNATOR FUNCTION
-Check each alternator
-Switch Front OFF (Verify the rear alternator picks up the load and
Warning Light) then, back ON
-Switch Rear OFF (Verify the front alternator picks up the load and
Warning Light) then, Back ON

 CABIN PRESSURE CHECK
-Take Plane upto 17,500 ft and ISOLATE each engine and check to see if
the cabin pressure can maintain max differential pressure (3.25psi)

 AUTO PILOT
-Verify Altitude HOLD
-Verify Heading HOLD

 NAVIGATION
-VERIFY VOR / ILS functionality

You don't need to go to 17.5 to verify the pressurization system. You could do it at 8.5, just be sure the differential gauge on the pressurization control panel is at max.

That said, taking the plane to 17.5 will expose bootstrapping, weak mags, and weak wastegates/turbos. Its well worth it to fly that altitude during the prebuy if at all possible! If one engine can't hold manifold pressure or starts to miss up there, you know you have an issue.

These airplanes were designed to fly high. They are both faster and more efficient the higher they go. I find 17.5 to be a "sweet spot" on a lot of trips. You're VFR, but with flight following at that altitude, you get great service from controllers, almost as if you were an IFR flight. Also, there is very little traffic at that altitude. I'll go hours without ever seeing another aircraft sometimes. My father once complained that it was "boring" flying so high because he couldn't see anything, so I flew him from Nashville to Dallas at 2500 ft - he loved every minute of it! Kept me out of a strong headwind too ;)

Thanks for all the info... Im kinda nervous and excited as this will be my first plane I own...the skymaster has always been my childhood dream plane...and now I can afford one...I hope it goes well Monday.

Luis

Best wishes for Monday. :D

Remember, make the decision with your mind, not your heart. It can be very hard to do in these types of situations, but you will be better off.

If you do buy it, you can always get great support here.

I understand...As much it will hurt, I am willing to walk away.

Max pressure differential on the P337 is 3.35psi. Should hold a 10000ft cabin at FL200 with either engine running. Check the flap cables, they make a sharp bend and the damage is hard to see, Cessna put out a SB on this a while back. Use every radio, autopilot and piece of avionics and make sure they all do what they are supposed to do. Make sure the pitot heat works, hard to find replacements.

Let us know how it went today! :cool:

Well did you buy it or not?

After all the help we gave you, we want to know what happened! :confused:

Yes! She is mine now. I just flew in last night and abit tired of all the traveling to get to her and then the flight back home in her was exhausting.

The flight was uneventful as it should. There are a couple of quarks to fix here and there....but the main thing, it didnt hold cabin pressure. I did notice that one of the dump valves (rear) was stuck and didnt want to move so hopfully thats the cause, but the front dump valve slide in/out very smoothly. I will be looking at her this weekend.

She cruised at 165-170kts all the way home.

I do have one question, and I think I know the answer.. I was flying at 10,000ft and when I pulled back on RPM for the front engine and noticed the Manifold pressure drop a few inches... I did this to the rear engine and the MP held perfectly.... could it be the turbo?

Firstly, congrats on owning a P337! :)

I hope you got some consideration from the seller because it won't hold pressurization. I hope its just a dump valve, but I doubt it. Keep us informed.

The rpm situation on the front motor indicates one of three things:
1. Weak turbo
2. Weak/misadjusted wastegate
3. Intake leak

I'm sure there are other possibilities, but chances are good you'll find your issue in one of the above.

Enjoy your new airplane and don't forget: PICTURES OR IT NEVER HAPPENED!!! :p

Here she is after my flight home.

Nice! :D

Looks like it has intercoolers. You're gonna want to check all that intake piping for leaks on the front engine.

I attached the flight records from last night. Your speed claims are backed up by the data. Looks like you didn't get out of there until pretty late. I guess you got to test out all the lighting!