Yes, I think many weather stuff are very temporary and you will notice it, except moutain waves you can fly for 60nm along its smooth lift/sink bars before noticing ;)
Without power one will notice it very well…but personally I did not encounter that many in the flat land
Vertical airflow component won’t change the IAS, unless you are changing the pitch for some reason eg. an autopilot in ALT hold mode.
Peter wrote:
Vertical airflow component won’t change the IAS, unless you are changing the pitch for some reason eg. an autopilot in ALT hold mode.
That is exactly what happens: you are flying along happily in alt hold and your IAS goes down by 10kts for no apparent reason. 5 mins later the opposite happens as you go on the other side of the wave.
Compound that with LOP turbo operation above critical altitude and suddenly you are chasing your airplane all over the place…ask me how I know…
And no, unless you are expecting it the reason is totally non-obvious, as it may well be totally exempt from turbulence or even nearby mountains.
It’s actually quite obvious if you take a look at the attitude indicator.
Dave_Phillips wrote:
It’s actually quite obvious if you take a look at the attitude indicator.
Why would this attitude change be significantly different from an identical airspeed loss caused by say an uncommanded gear door opening or a loss of engine power?
Thinking again, you are right, even if you increase power to maintain indicated airspeed, 500fpm downdraft at 100KTAS means you would need arctan(5%)=3 degrees extra pitch
You have to add attitude increase due to airspeed loss.
The thing that makes it obvious is the cyclical nature. Obviously this depends on airspeed, the wavelength of the wave and the angle you are crossing, but it is typically a few minutes.
I must say that if I suffer a loss of airspeed, I first check for ice, and then assume mountain wave. Only if there is no ice and the performance loss continues more than about 5 minutes would I start considering other things.
As I say, it is the explanation for the vast majority of such experiences.
Timothy wrote:
I must say that if I suffer a loss of airspeed, I first check for ice, and then assume mountain wave. Only if there is no ice and the performance loss continues more than about 5 minutes would I start considering other things.
Sounds like a plan! I also check flaps since sometimes they have been inadvertently partially extended.
I think we have all at some point left the flaps in the takeoff position, and I even did it with the landing gear a couple of times 
But more subtle reasons can be a rigging problem e.g. the flaps not fully up on one side, and you need only a few mm to then require a lot of aileron trim, and/or rudder trim, and lose perhaps 5kt, and the chances are that almost nobody in the maintenance business will ever find it.
Other, easier to fix, reasons can be a duff RPM gauge (on a fixed pitch prop plane), a duff fuel flow gauge, etc.
After reading this thread, I pulled the data (audio, video, track log) of my recent safety landing for review, conditions: VMC, CAVOK, M01C/M03C, cruising at altitude, autopilot on track, no auto-trim, fixed prop, me reading something else. About 2 hours in flight, RPM starts dropping from 2450 to 2200, speed starts to drop immediately due to AH, time to react was less a minute, autopilot disengage after 2 minutes, remaining power assisted glide was plenty to safely reach the next airfield. From the material I suspect everybody will recognize fast a quick 10 percent drop in performance in a fixed prop aircraft. VP/CS, auto-trim and slowly degrading performance is a different animal, but a 10 knots reduction should be recognized immediately (Btw, on IFR flights I put these little PostIt arrows at meant positions on the instruments to increase situational awareness – anybody else?).
speed starts to drop immediately due to AH
What is “AH”?
