Thu Apr 25, 2024 11:39 pm
#2018919
On my last flight, with a very experienced instructor friend beside me (a refresher flight after me being grounded for while), the stall warner was intermittently sounding at our Jodel's normal climb speed of 130kph (yes - the French use kilometres per hour).
With a stated stall speed of around 110kph, I have had the warner bleep on thermal days at higher speeds, so this didn't surprise me too much, but this was more persistent than normal and it wasn't particularly thermal.
Since our plan was for a few circuits to blow the cobwebs away, we continued in the circuit as normal. I have become accustomed to cruising at higher speeds due to running in a few engine, so we discussed the stall warner and continued at a fairly quick pace downwind - aircraft seemed to be performing well. On base and final I again got the stall warner bleeping intermittently when going at faster than traditional approach speeds (keeping speed up to protect new engine) but as I slowed at short final I felt a slight buffet so increased speed / added power before landing without floating (despite higher listed speed).
Continued to complete the touch & go and opened the warner discussion on climb out. Always helps to have an experienced test pilot beside you so using excellent CRM I asked him to have a feel. He quickly confirmed the buffet at 130kph was an actual approach to stall and using alternate air made no difference, so we completed the circuit using an approach speed of 130 * 1.3 to an uneventful landing.
With an aircraft appearing to stall at 20 kph more than it should, we were a bit worried about the aircraft (it would have been very easy to have ignored the warning horn, tried to fly the numbers and ended up stalling).
We put the ASI though a calibration process on the ground and it read correctly.
So have you guessed the cause?
Simply the static hose had come out the back of one of the instruments so the ASI was operating on cabin air rather than its proper static source.
When switching to an alternative static source, I've thought about attitude under reading (perhaps 20 feet so not a big deal when VFR) and assumed there would be some minor speed change but I've never quantified it. In our Jodel, it causes the ASI to overread by about 22kph at stall speed. Pilot be aware!
With a stated stall speed of around 110kph, I have had the warner bleep on thermal days at higher speeds, so this didn't surprise me too much, but this was more persistent than normal and it wasn't particularly thermal.
Since our plan was for a few circuits to blow the cobwebs away, we continued in the circuit as normal. I have become accustomed to cruising at higher speeds due to running in a few engine, so we discussed the stall warner and continued at a fairly quick pace downwind - aircraft seemed to be performing well. On base and final I again got the stall warner bleeping intermittently when going at faster than traditional approach speeds (keeping speed up to protect new engine) but as I slowed at short final I felt a slight buffet so increased speed / added power before landing without floating (despite higher listed speed).
Continued to complete the touch & go and opened the warner discussion on climb out. Always helps to have an experienced test pilot beside you so using excellent CRM I asked him to have a feel. He quickly confirmed the buffet at 130kph was an actual approach to stall and using alternate air made no difference, so we completed the circuit using an approach speed of 130 * 1.3 to an uneventful landing.
With an aircraft appearing to stall at 20 kph more than it should, we were a bit worried about the aircraft (it would have been very easy to have ignored the warning horn, tried to fly the numbers and ended up stalling).
We put the ASI though a calibration process on the ground and it read correctly.
So have you guessed the cause?
Simply the static hose had come out the back of one of the instruments so the ASI was operating on cabin air rather than its proper static source.
When switching to an alternative static source, I've thought about attitude under reading (perhaps 20 feet so not a big deal when VFR) and assumed there would be some minor speed change but I've never quantified it. In our Jodel, it causes the ASI to overread by about 22kph at stall speed. Pilot be aware!
WalesFlyer50 liked this