Pocono 470T Overboost Update

This Update is for the twin engine (piston) Piper Pocono, which adds two annunciator lamps to the panel, to warn the pilot of manifold “overboost” conditions, found mainly during take-off situations.  

Performance flights of the Piper Pocono 470 twin 8-cylinder 720 Cu-In 400 HP airplane and the 570 quad-engine 6-cylinder 540 Cu-In 260 HP engined airplane, I actually found that the 470 should have the designation of a 470T (turbocharged) engine while the 570’s remain as they are, non-turbocharged, making them designated 570 as they would have been flown if the 570’s would have been producted.  

Before you make the simple gauge additions to the 470, check out what the manifold pressure gauge does on a full-power take-off, compared to the four-engine, full-power application.  Too high a manifold pressure, I saw that I had little to no reactions to my Prop pitch changes on the 470, yet the 570 responded as I expected.  A bit of research, I found a Service Bulletin 592, so I looked for and found these annunciator lamps for this airplane, as long as I didn’t allow the lamps to light, I had prop pitch control!  However, I could not apply full throttle as quickly as I have done all my hours flying the XPA35-470 Pocono.

I believe, very few of us have seen this “boost event” since so few of us had an “altitude engine” available to us to manage.  Going from the turbocharged Piper Arrows and coming back over to the Pocono, I had no knowledge of the possibility of different Overboost levels and hence the annunicator lamp update. They will now warn me of the rapid “spin up” of the 35 InHg limit which in real life, if I exceeded that for as little as two-seconds, I would have to make an engine maintenance logbook entry of said event.  According to the Service Mandated Lycoming “Overboost” Bulletin, applies to all turbocharged piston engines, that do not have a “relief valve?”  These lamps will light 2-inches before the maximum level specified in the AIRCRAFT.CFG file, which still match the 400 HP Comanche IO-720 engine of the time.

 Pocono Update by Ronald Prindle