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208 crash KPLN Jan15/012 flightawareWX/Sat shows wave cloud
http://www.caravanpilots.com/phpBB3_archive/viewtopic.php?f=1&t=825
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Author:  pdw [ Fri Oct 07, 2011 6:01 pm ]
Post subject:  208 crash KPLN Jan15/012 flightawareWX/Sat shows wave cloud

It looks like agitated wavecloud (Undulatus Asperatus .. invented in 2009) on that satellite image which is associated with the flight's data on flightaware. The link to this particular flight is listed in the Aviation Safety Network database for Jan 15 2012, the Martinair Cessna Caravan.

Author:  Tailwheel [ Mon Oct 10, 2011 12:06 pm ]
Post subject:  Re: 1008hPa is common pressure (Accident Links)

Congratulations Pete,
[b]Nice to see your trying to keep this website going, but I'm afraid it's not being used to advantage by most pilots.

Hope that changes, Ciao
[/b]

Author:  pdw [ Mon Oct 10, 2011 2:36 pm ]
Post subject:  -

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Author:  pdw [ Thu Mar 29, 2012 5:53 pm ]
Post subject:  Gravity Wave action described/understood

As pilots relate accounts of gravity wave (gravity current) experiences, it tells of their unexpected affect on their flight/flight-path's (stability of the flight parameters). Understanding these waves is also seeing how confusing their affect will be to that pilot seeing a significant encounter or 'wave train' for the first time, then having to deal with tricky/rapid changes in vertical speed and airspeed that are propagating with too-little or no warning.

Wavelengths vary in length & height as well as in 'approach direction', the angle by which they meet a flightpath. Visible evidence of g-wave action can be seen on timelapse video ' Tama Iowa May 6 2007 ' on youtube, a golf-course camera angle that recorded passing g-waves in their wave-train with each wavelength visible, their cloud-alterations over the 40 minute period of video from 7:59AM. Gravity currents are occasionally born in advance of a cold front, where following (within) a Lo's warm-sector, where a 'low-level jet' perpendicular to the frontal action becomes the trigger (like a "pebble in a pond") as stable low-level "duct air" has moved in at the surface (propagates "an undular bore" or 'wave initiation'). The 'wave trains' will have less cloud presence (so that individual g-waves will not be as visible in their oscillations) between an upward motion toward condesation/cloud and then back down into subsidence/no-cloud (their rise and fall of the 'amplitude') when occurring between low-moisture airmasses.

The g-wavelengths apparently can be anywhere between 5nm up to 300nm where the shortest are noticeable as they create the sharper ripples in a flightpath to produce a more-detectable 'wave-oscillation turbulence'. Longer g-wavelengths (smooth across the flightpath) can easily become an unexpected Decreased Performance experience with their gradually-entered/less-noticeable and decreasing VSI/IAS, but a more substantial shear-effect/airspeed-decay will begin where negative shear is also present when the aircraft's track crosses a shear transition between the two different airmasses at the front. Encounters in approach might require some airspeed reduction as ASI spikes suddenly/quietly higher on the upside/beginning of an oncoming wave (so the pilot reduces power) but where next the descent-IAS stabilizes at the wave-peak, and finally might require extra nose-down/power on the wave's backside/downside where airspeed decay escalates again. The plummetting VSI during 'decay' introduces an unanticipated & precarious sink rate particularly to the landing descent, one that eventually commands strong G-force pitch-up action to re-establish pitch-axis at low AGL. Early seconds of decay-onset could be too-easily overlooked in a 'subtle airspeed-loss', and even further on in that surprisingly-rapid progression into steeper decay of any 'smooth entry' that transpires deeper into 'decreased performance'; it's not felt nor seen (between IAS scan intervals) as the 10-12 knots too-quickly escape pilot detection. A lot to lose off a slow climb/descent AS of the slower single fixed-wing plane, undetected.

Maintaining steady speed and pitch axis on descent or climbout: At which point in a normal 'airspeed scan-interval' is a 'smooth IAS decrease' to be noticed when onset is rapid and turbulence (feel) is not present as an indicator ? "Rapid" would be something like 'ten knots of IAS lost in five seconds' on a stable pitch/IAS, which is six times the airspeed loss-rate as 'five knots lost in ten seconds', where 'ten seconds' is also the scan-interval that masks this 'high rate' of loss in between two scans of the ASI (the lost airspeed escapes attention entirely in mere 'ten seconds' of 'eyes-off ASI' while in smooth descent through air that seemed so stable).

A low rate of decay (5 kts in 10 sec) is even recognized very quickly with its associated/identifiable 'bit of turbulence' generally present to alert attention to the performance loss. For that reason most might get accustomed to the see/feel indicator (bumpiness alerts to the extra IAS check) over a time and be led to believe any shear-loss of airspeed is as 'easily recognizable' every time, believing the more significant/high-rate shears (10 kts in 5 sec) would logically lend themselves to be recognized the best. So it's perhaps understandable some get fooled into thinking the aircraft 'lets you feel' the shear in all conditions, or that a smooth shear-off of the IAS is never rapid. But it was with the associated ice-catches where the silent decay-rate most-often has given opportunity for coarsening ice textures on laminar surfaces to quickly escalate a less noticeable performance loss into a disaster (using analysis of many ice-accident reports where neg shear effects are involved).

It may be nearly impossible to ANTICIPATE a rapid/unexpected 'high rate' with little or no 'feel' to it, esp with other distraction; nor is it ever certain to CATCH the silent/non-turbulent Xtra-low climb/descent speed that dropped fast from one moment to the next. 'Such' AS-deficiency event arises where the descending/climbing (fixed wing) aircraft manages to cross through a whole horizontal distance of 'smooth negative shearzone' within the common AS-scan interval. So even a low-AS / stall warning is at the point of being too tardy as the AS has already decayed far and/or might still be in further fast-decay (with risk of an untimely response-delay in the confusion) at the surprise-moment it is heard or seen (audible or flashing).

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