REFLECTOR: Cruise Mystery

Sun Feb 15 19:38:34 CST 2009

Chuck I would like to check out your data on my GRT and see how it all indicates.  I have never see numbers like this in my XL but I would like to watch them on mine, with your data.   I don't run mine at LOP but have at peak.  I have an IO-540/260  I do see around 130 kIAS at 10.5K burning around 11.8gph.  20 MAP, 2300 RPM. I have a single GRT with the stick so I can read it. 

Dave Dent
XL/RG N32XL 
  ----- Original Message ----- 
  From: Chuck Jensen 
  To: Velocity Aircraft Owners and Builders list 
  Sent: Sunday, February 15, 2009 5:48 AM
  Subject: REFLECTOR: Cruise Mystery

  Okay, here's one that hasn't given up its answer easily.  Last week, I make a quick trip over to 2W6 (St. Mary's Regions, just south of DCA) from Knoxville, TN TYS in the Velo XLRG.  The front of the wx system that tore up Oklahoma on Wednesday had just passed the night before and the low pressure was parked over NY (wx that brought down the Continental flight).  At cruise at 11,000' my ground speed was 254kts while my IAS was 161 kts.  I was cruising on the TruTrack III GSGV A/P, tracking the GPSS route with Alt-Hold engaged.  MAP was 20.1, RPM 2400 and fuel flow 11.7 gph.

  Over the TRI area, I noticed the IAS start to slowly drop.  I checked all instruments and everything was nominal and there was no pitch or tone change from the engine.  The speed continued to bleed off until IAS was 113 kts.  The A/P had indicated it needed more nose-up trim to keep it in range.  The slow speed was confirmed by a pitch up about 5-6 degrees.  I thought it might be the Grand Rapids dual EFIS indication that was screwed up but I confirmed the airspeed on the backup mechanical ASI and the pitch was confirmed on the independent backup MCI LifeSaver AI.

  Observable measurements.  The oil temperature had increased from 198F to 225F, the CHT had increased 50F-70F and the EGT were all much higher...where normally they run in the mid to upper 1400F, they had increased to low to mid 1500F.

  I had been running WOT because of altitude as well as running LOP.  I enriched the mixture and could hear just a minor increase in engine output, even though the RPM and MAP didn't change.  The increase was the same as when you've been running LOP and have descended a few thousand feet and then enrich the mixture.  

  Though I don't know that the slow but steady resumption speed coincided with the enrichment, the speed did increase after a few minutes and climbed to 184 kts IAS and stayed there a few minutes before settling back to the 160 range. (Note, this could not have been speed gain from altitude drop because I was steady 11K.  One would have expected the speed to increase and slowly achieve the original 160 kts, but it overshoot and actually went 10-15 kts higher than normal cruise).   5-10 minutes later, this cycle repeated itself, though the drop was to about 144 kts IAS, and then it would pick up again to an IAS above my normal cruise and then settle back again---all without change in MAP, RPM or fuel flow.  Each time, there was a corresponding change in CHT and EGT, though not as pronounced as the first episode.

  While I wanted to check the L/R ignition, I was over rough terrain and with ground wind in most areas G40, I didn't want to have to try to make an emergency landing if turning either ignition systems on/off  (Rose electronic on L, mag on R).  On the return trip that afternoon, the winds at altitude were >90 kts.  Fortunately, they were quartering from the WNW and I was headed 250.  Still, I had to turn 20-27 degrees into the wind to hold my heading---a new record.  I couldn't go lower than 10K as the mountain turbulence would not have been bearable...at least for the weak stomached, myself included.  There were several speed excursions on trip home, but none as extreme as the first one on the way over. 

  Over the TriCities airport on the return trip, most of it at 115 kts GS, I cycled the two ignition systems.  The mag was quite rough but the plane would have kept flying adequately.  When only the Rose ignition was running, it was quite smooth, but not perfectly so.  In short, both ignition systems were operating, though whether they were optimal I don't know.  So, I'm trying to sort out how these unaccountable operating changes could occur with no apparent changes in power setting or plane configuration.  

  Impressions and first thoughts.  There was no configuration change in the plane during all of these excursions, though there was the obvious pitch up as the speed slowed and a pitch down attitude as the new higher than normal cruise speed was experienced, but no changes to the power settings.   It seems there was a distinct change in power output from the IO-540 300, even though none was apparent from any audible change in the engine.  My suspicions center around the ignition system, primarily because of the changes in EGT (the high oil temp may have been reduced cooling as speed bled off and pitch up reduced cooling) that tracked the power changes.  With the apparent drop in power output, the EGT would steadily climb.  Since mixture is not being changes, I assume this would indicate a retardation of the ignition spark...leaving more fuel to burn post-cylinder, resulting in less power produced in the cylinder and more of the fuel burned as it left the cylinder, showing up as higher EGT temps---all while fuel flows remained essentially the same.

  Since I'm not an authority on ignition systems (nor much of anything else), I'm not sure that I understand the mechanism that would cause the timing to shift back and forth in this manner.  I would love to have the increased cruise speed that I would see for a few minutes at a time, but I'm not down with the power drop on the other side of the excursions.  Of course, perhaps it's not the timing at all, but I'm at a lost to see what else could cycle power and EGT in this manner.

  So, I'm stumped as to where to look and what to address.

  One fortuitous happening....I just happened to be recording the GRT EFIS data on a USB stick, so I have the event captured on the stick.  I later captured 2 or 3 other excursions as well, so anyone that is able to look at the GRT demo recordings, I'd be happy to email them.

  Chuck Jensen 

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