G5 airspeed disagrees with steam gauge by 5+ kt, how bad is this?
Posted by drdsheen@reddit | flying | View on Reddit | 24 comments
I was doing a flight in an aircraft I haven't flown in for a long time and, on climbout, my instructor was saying "speed speed nose down speed" and I was very confused because I was at 80 knots. Then on downwind I saw the G5 was about 5-10 knots higher than the steam airspeed indicator.
Obviously this is a problem because that much speed difference is a lot, especially close to stall. Is this a "ground the plane" issue?
Captain_Flannel@reddit
I've installed many many G5s and GI 275 and all sort of digital airspeed indicator. They almost always read lower than the analog airspeed indicator. Analog airspeed indicators always read high by a few knots or mph.
I would definitely squawk it and let an avionics shop test it. I suspect there is a big pitot leak, or something restricing flow in the tubing.
Ok-Money2811@reddit
Look for a source of wind direction on the ground like smoke, lake ripples, big flag, etc
Fly low…legally…directly into the wind at a constant airspeed, note the groundspeed on the GPS. Turn around exactly 180 degrees and fly exactly the same indicated airspeed, note the groundspeed again. Add the two ground speeds and divide by 2, see which instrument is closest to the result.
The math result will be TAS but low and slow it will be close enough to IAS to figure out which is right. The lower you are the better so find an open field and do it at 500ft if you can.
the_devils_advocates@reddit
I did this on mine years ago... shocker, the 60 year old analogue ASI was the incorrect one. I'm guessing thats the case most of the time
tailwheel307@reddit
The G5 is not the certified source of airspeed data for the purpose of certification. Use the analog.
With that said I would generally want a less than 5% difference between indications. This could be fixed the next time the aircraft is in the avionics shop for altitude encoder certification where they can calibrate the analog and G5 airspeed indications.
Overall_Raisin_9159@reddit
Ask me how I busted my instrument oral.
mcweaselcop@reddit
Last I did one, only the altitude was calibratable on the g5. If the airspeed was truly wrong, then it needs sent back to garmin
tailwheel307@reddit
After I posted my comment I went digging through tolerances for airspeed indication and the EASA standards (which reference the FAA standard) were eye opening.
I-r0ck@reddit
I’ve never worked on the G5 but Garmin’s GSU 75 works that way. It comes from the factory calibrated and if it’s wrong it has to get sent back.
Thats_my_cornbread@reddit
Yea this is what annoys me about G5’s. Every one I’ve flown has disagreed with the analog by varying margins through out the envelope.
The fact that it’s not primary means you can’t get rid of the analog. So there has to be two conflicting airspeeds. I wish you could just select it “off” and declutter the screen.
Then there’s the gi275, legal as primary, but they jammed it into a smaller screen so it’s still kinda useless.
Garmin really dropped the ball on these two in my experience.
drdsheen@reddit (OP)
I would love so much to be able to turn off the non-primary airspeed and altitude.
Jwylde2@reddit
No it’s not a ground the plane issue.
zeropapagolf@reddit
Is the G5 set to MPH? I flew a 172 once where that was the case. Easy fix in the configuration menu.
If not, an avionics shop can calibrate the G5.
Jwylde2@reddit
No, they cannot calibrate the airspeed. Only the altitude. It will need to be sent into Garmin to have the airspeed recalibrated.
TheVillianOfValley@reddit
Analog airspeed indicators lag. Do the two indications agree once you’re in steady cruise?
Analog airspeed indicators also wear out. ADAHRS units have no mechanical parts and do not wear out. I’d trust the G5 before trusting the analog ASI, even though the G5 air data is legally secondary per its STC.
92milkman@reddit
Same on ours in the 172, 5 mph difference at a minimum when in the pattern. G5 shows faster. Both are set to mph.
nhorvath@reddit
double check the fast one isn't in mph. if that's not it use the slower one. or do a power off stall and note which matches the published stall speed.
highflyer10123@reddit
Pretty common. You can take any two instruments. Put them side by side with the same inputs. They will all show slightly different. For example flip through your altimeter certifications for when the avionics shop does the pitot static cert. it will tell you the error at all altitudes. Now compare it to a different plane. You will see the errors are different. Same concept goes for airspeed indicator.
Impressive_Jury_2211@reddit
Look into ADC and AHRS. Will improve your knowledge and confidence.
Fight_Or_Flight_FL@reddit
I fly a half dozen planes regularly that don't have exactly same airspeed or altitude on the glass vs the analog. I tend to trust the analog more when I'm slow.
mtconnol@reddit
My money is on: G5 set to MPH, or static tube is detached from the back. If it doesn’t change when you select Alt static, then you know. If it is set to MPH it will say so in small print near the airspeed tape.
makgross@reddit
It’s not the static port or altitude would also be wrong.
RyzOnReddit@reddit
Could be worse, could be better. I’d fly whichever one is indicating slower when you’re near the ground.
makgross@reddit
I’ve seen wrong G5s too many times….
But without an avionics shop, the best way to tell is to stall it. At altitude, of course.
If you’re really 5-10 knots fast, you’re also going to float a lot on landing.
rFlyingTower@reddit
This is a copy of the original post body for posterity:
I was doing a flight in an aircraft I haven't flown in for a long time and, on climbout, my instructor was saying "speed speed nose down speed" and I was very confused because I was at 80 knots. Then on downwind I saw the G5 was about 5-10 knots higher than the steam airspeed indicator.
Obviously this is a problem because that much speed difference is a lot, especially close to stall. Is this a "ground the plane" issue?
Please downvote this comment until it collapses.
Questions about this comment? Please see this wiki post before contacting the mods.
I am a bot, and this action was performed automatically. If you have any questions, please contact the mods of this subreddit.