Randy
1972 PA32-300
- Joined
- Aug 3, 2012
- Messages
- 74
- Reaction score
- 22
I have a cautionary tale. In the last year we installed a GFC 500 autopilot and two G5s. Terrific units, the system works great. Very stable and flies beautiful multi waypoint flight plans and approaches. We probably have 100 hours on the system.
I just flew a long cross country (1000 miles). On the way home, I made two bad landings. On both approaches, I flew, or tried to fly, my standard 90 MPH (old airspeed indicator) pattern speed and slow to 80 MPH over the fence. Both times, the airspeed slowed faster than usual and I ended up apparently slow on base and final. I added power and trimmed nose down, to no real effect. Also, the nose became heavy. I've flown this airplane for 25 years, so I am very comfortable with it and very familiar with the "feel." It felt wrong. Both times, I landed long and used too much runway. I don't recall any warning lights or aural indications of any issue. But I was concentrating on flying the plane and landing.
I figured something was amiss, but was not sure what. I thought maybe it was a CG issue. These were long legs, burning 65 out of 84 gallons. But CG would move aft with fuel burn, so the result should not be nose heavy. And I've flown lots of long flights without this being an issue, so I ruled out CG. Maybe one of the new AP servos was hanging up, adding extra control forces? I figured it out this past Monday when I took the airplane to the avionics shop for an unrelated repair. On take off, the AIRSPEED indication climbed to 100 MPH and basically stopped. A 300 HP Six with one person, even with full fuel, will climb at 1500 FPM and accelerate briskly. After leveling off, the airspeed on both the mechanical instrument and the G5 stayed around 100 MPH. I realized, finally, that I was looking at inaccurate airspeed. Those bad approaches were bad because I was getting bad airspeed data and actually flying much faster than the indication. And because the G5 also "thought" the airspeed was low, the envelope protection system in the GFC 500 was adding down force on the controls, making the nose heavy. All the extra power and down trim I added did not correct the bad airspeed shown by both instruments. Because the same wrong airspeed was shown on both the airspeed indicator and the G5, I knew it was something in the system, not the gauge.
I flew the approach to the avionic shop airport by power setting, attitude and experience rather than airspeed (which was still wrong). I pulled the circuit breaker on the GFC 500 and held down the red AP disconnect button to disable the envelope protection and made a normal landing. When I explained the problem to the shop, they said either the pitot was blocked (it wasn't) or there was a huge leak in the pitot static. It turned out that was exactly right, there was a hole in one of the new pitot hoses, probably a result of chafing. New hose, carefully tie wrapped away from everything else and problem cured.
The real confusion here was due to an airspeed indicator(s) that were working but wrong. If the airpseed was zero, that would have been obvious, The insidious issue here was airspeed that was somewhat working and yet wrong. And it took me too long to figure out the problem.
Anyone flying with the GFC 500 and G5 should recognize when the system is trying to keep you safe by adding down or up forces. If there is an indication of that on the G5, i didnt notice it. You should also know how to disable the system if it tries to help when it should not. You can temporarily turn it off with the red button. You can also go into the menu on the G5. Or you can pull the CB, but you need to know which one to pull.
I just flew a long cross country (1000 miles). On the way home, I made two bad landings. On both approaches, I flew, or tried to fly, my standard 90 MPH (old airspeed indicator) pattern speed and slow to 80 MPH over the fence. Both times, the airspeed slowed faster than usual and I ended up apparently slow on base and final. I added power and trimmed nose down, to no real effect. Also, the nose became heavy. I've flown this airplane for 25 years, so I am very comfortable with it and very familiar with the "feel." It felt wrong. Both times, I landed long and used too much runway. I don't recall any warning lights or aural indications of any issue. But I was concentrating on flying the plane and landing.
I figured something was amiss, but was not sure what. I thought maybe it was a CG issue. These were long legs, burning 65 out of 84 gallons. But CG would move aft with fuel burn, so the result should not be nose heavy. And I've flown lots of long flights without this being an issue, so I ruled out CG. Maybe one of the new AP servos was hanging up, adding extra control forces? I figured it out this past Monday when I took the airplane to the avionics shop for an unrelated repair. On take off, the AIRSPEED indication climbed to 100 MPH and basically stopped. A 300 HP Six with one person, even with full fuel, will climb at 1500 FPM and accelerate briskly. After leveling off, the airspeed on both the mechanical instrument and the G5 stayed around 100 MPH. I realized, finally, that I was looking at inaccurate airspeed. Those bad approaches were bad because I was getting bad airspeed data and actually flying much faster than the indication. And because the G5 also "thought" the airspeed was low, the envelope protection system in the GFC 500 was adding down force on the controls, making the nose heavy. All the extra power and down trim I added did not correct the bad airspeed shown by both instruments. Because the same wrong airspeed was shown on both the airspeed indicator and the G5, I knew it was something in the system, not the gauge.
I flew the approach to the avionic shop airport by power setting, attitude and experience rather than airspeed (which was still wrong). I pulled the circuit breaker on the GFC 500 and held down the red AP disconnect button to disable the envelope protection and made a normal landing. When I explained the problem to the shop, they said either the pitot was blocked (it wasn't) or there was a huge leak in the pitot static. It turned out that was exactly right, there was a hole in one of the new pitot hoses, probably a result of chafing. New hose, carefully tie wrapped away from everything else and problem cured.
The real confusion here was due to an airspeed indicator(s) that were working but wrong. If the airpseed was zero, that would have been obvious, The insidious issue here was airspeed that was somewhat working and yet wrong. And it took me too long to figure out the problem.
Anyone flying with the GFC 500 and G5 should recognize when the system is trying to keep you safe by adding down or up forces. If there is an indication of that on the G5, i didnt notice it. You should also know how to disable the system if it tries to help when it should not. You can temporarily turn it off with the red button. You can also go into the menu on the G5. Or you can pull the CB, but you need to know which one to pull.
Last edited:
