Airspeed Difference

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DiamondMike
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Airspeed Difference

Post by DiamondMike »

2007 DA40 with Garmin G1000 WAAS.

In cruising flight PFD indicates 115 KIAS. Standby airspeed indicator shows 95 KIAS.

Anyone seen this before?

What could be wrong?

Thanks.
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Steve
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Re: Airspeed Difference

Post by Steve »

Partially blocked pitot line? I am not familiar with the DA-20, but doesn't it usually cruise faster than 115?
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Re: Airspeed Difference

Post by DiamondMike »

This was a DA40 and this is just one data point.

Hard for me to believe a blocked pitot since Garmin G1000 showed accurate airspeed. I think there is only one pitot source under the left wing.
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Steve
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Re: Airspeed Difference

Post by Steve »

DiamondMike wrote: Tue Sep 03, 2019 11:10 pm This was a DA40 and this is just one data point.

Hard for me to believe a blocked pitot since Garmin G1000 showed accurate airspeed. I think there is only one pitot source under the left wing.
If it is a DA40, then an IAS of 115 is really slow for cruise. My 2001 (older and slower) cruises at 122-123 KIAS at 5500 ft (2200 RPM and 65%). The pitot line could be blocked or pinched anywhere, including between the G1000 and ASI.

Steve
Last edited by Steve on Wed Sep 04, 2019 12:35 am, edited 1 time in total.
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Rich
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Re: Airspeed Difference

Post by Rich »

115 in cruise would be OK for low power/high altitude, but otherwise pretty low. I would suspect a leak in the pitot line - loose fitting, cracked tubing, etc. A pitot/static system test every 24 calendar months is required for IFR flight, anyway. I suggest you check into when this was last done and see about a recheck.
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DiamondMike
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Re: Airspeed Difference

Post by DiamondMike »

The 115 KIAS data point occurred while on an instrument approach, not cruise flight.

Thanks for responses.

I will have pitot system leak checked.
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Re: Airspeed Difference

Post by Antoine »

Mike you can start by looking at the pitot static assembly under the left wing.
There could be a bug nest there. static port is on back side.
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Ian Sage
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Re: Airspeed Difference

Post by Ian Sage »

If your pitot check comes back good you may consider having one or both instruments calibrated. While a spread of 115 to 95 is excessive it is not unheard of for two gauges to display very different indications.
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Rich
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Re: Airspeed Difference

Post by Rich »

Antoine wrote: Wed Sep 04, 2019 12:09 pm Mike you can start by looking at the pitot static assembly under the left wing.
There could be a bug nest there. static port is on back side.
While the orifices (3 of them) on the pitot-static blade should routinely be checked for obstructions, the fact that the two instruments read differently suggests a problem downstream of this single source.

In addition, since this problem shows up on approach, it's less clear which one is "right" and now puts problems with the static lines more likely to be involved. Are the two instruments in synch in any phases of flight?

Of course, the worst case would be a problem with one of the instruments itself. $$$$$ :(
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Re: Airspeed Difference

Post by Antoine »

Yes Rich, what you say makes sense.
I had the following experience with my aircraft which has 2 x dual static ports (total 4), with a cross tie. Some bug got this and produced some rather horrible slime.
The first instrument to show distress was the analog "round" backup altimeter which started swinging like it did not have enough damping . A few weeks later the G500 speed stripe started showing values that did not make sense with the attitude of the aircraft. I then finally managed to get my old brain moving and it took my lazy body to the static ports which I had neglected in my preflight. Not anymore...
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