gallois wrote:
Could the answer be much more simple.
IAS is a result of the difference between total pressure as collected at the pitot and static pressure at a port out of the wind.
Pt -Ps =Pd (dynamic pressure)
As you travel through different air masses and different wind directions the Pd is going to change.
I’m not sure that’s a simpler explanation…
In any case, if the air masses have so different density that the IAS will change by 20% or more, then you would have a very noticeable frontal situation. It can’t be windshear either as that would only have a momentary effect.
Speed fluctuations like this can only be caused by up/downdrafts. Either waves or thermals. Waves don’t have to be mountain waves either. You frequently see weak but noticeable waves in and near inversions.
The wikipedia info is nice indeed. In particular the image below was very informative for me (fluid dynamics simulation, wind left to right, mountain in the middle – better description in this wiki article). I never realised mountain waves depart the range at a vertical angle of 45 degrees. Agree it fits with what I saw – mountain wave it is. I considered pressure variations as well but that doesn’t explain the GS changes.
Could the answer be much more simple.
IAS is a result of the difference between total pressure as collected at the pitot and static pressure at a port out of the wind.
Pt -Ps =Pd (dynamic pressure)
As you travel through different air masses and different wind directions the Pd is going to change. This can often be seen in a yaw situation or when a static port is blocked. This of course is only likely to occur when you have static ports on both sides of the fuselage.
The blocking of one of these ports can happen as a result of an very strong wind at one of the ports and causing a drop in pressure.
Turbulence is the result of different masses of air at different points on the aircraft.
Different air masses may well be at different air pressures.
@Tango in principal mountain waves can reach up to the tropopause. Gliders went up to around 50.000 feet in them. Depending a lot on whether there are stable and unstable layers in the atmosphere, wind and topography. It’s an interesting read, you can start in the Wikipedia, for example. If you were over the massif central, this goes up to 1.800 meters. It’s quite hard to estimate more without your actual position and an overlay of the topography that was below and in front of you (with respect to wind direction). So to me it looks quite obvious, however I don’t quite understand what questions are open.
@UdoR sure but (and please forgive my flatlander ignorance) I was at FL120 (GPS alt 13000 actually) so I didn’t think that would be a possibility-the only peaks high enough would be the ones to my east. And I thought mountain waves should be in the general direction of wind so hence my question regarding wind indication change.
@Tango if you were SW of LFLS then you were somewhere about right above Massif Central, see e.g. here for a topographic map of France.
With 35 knots and given the topography down there (valley of Mistral) you don’t need the alps to encounter waves.
If you had winds from N to NNW in that strength makes perfectly sense that you encounter waves directly overhead.
In February I was flying a 40 knots headwind over monstrous Spessart that has about 300 meters above surrounding (highest but isolated peak is 586 meters above main sea level). Flying in FL60 on autopilot we had fluctuations of about 25 knots in the airspeed. Because I have some wave soaring experience in gliders I spotted it right in the first wave. Wasn’t hard to understand, although I was surprised about the amplitude of speed change given the low terrain. My instructor said we should make a foto when the plane was fastest to send it to the plane owner to show him how we got the shit out of that old duck, that it flies so fast with us on typical settings. (he came across the reason also quite fast, he’s also instructor on gliders).
Why should the wind indication change? The wind has a constant speed in a wave. It’s a sinusodial wave that you’re flying along. When the wave is going down your autopilot is trying to maintain altitude and decelerates. When the wave is going up the AP tries the same and you’re speeding up. But the wind speed stays always the same.
Interesting. What do you think the effect should be on the indicated wind speeds on the G1000? I ask because I also considered mountain wave but figured the wind should then indicate coming from my east instead of almost purely head on (no mountains to speak of north of where I was)? At the moment of the indicated fluctuation I was more or less SW of LFLS on the track shown above, headwind 35 kts give or take with maybe 3 kts sidewind. Also, I didn’t (or don’t) understand why the wind indication did not fluctuate but instead stayed almost rock steady, but that’s perhaps due to the way it’s calculated hence my question. Perhaps there is a lag or latency built in the indication?
Tango wrote:
Not sure I fully understand what causes this
PROB95 Mountain waves.
I saw something similar (similar to some degree at least) flying north from the Mediterranean. IAS fluctuated similarly as the GS shown below, somewhere between 100 and 130 kts and back on a cycle of some 10s of seconds.
I’m not sure whether GS fluctuations sync with IAS perfectly – I don’t have better log data unfortunately. But I did look at headwind which indicated a stable 35 kts (yay Mistral 🙄) and I was at FL140 on autopilot with constant power setting (I did change power a few times to see effect on fuel efficacy but not in the circled part) in VMC. Not sure I fully understand what causes this. How is the wind data on a G1000 calculated?
@Maoraigh absolutely, and I think that is @dublinpilot’s main point. Typical moderate turbulence wind shear 15-20 knots can quickly take you from Va into the yellow caution speed range, even while holding a constant attitude and allowing altitude change to reduce airframe stress.
