A tragic accident occurred on Saturday, August 7th where a Mooney M20M / 257 TLS Bravo, plummeted to the ground as the flight approached the Flying Cloud Airport from the northwest near Minneapolis, Minnesota. The pilot and two passengers died in the crash. According to Flightaware the flight departed at 4:56 pm CDT (2156Z) from Chandler Field Airport (KAXN) headed on a short IFR flight southeast to Flying Cloud Airport (KFCM) liocated just to the southwest of Minneapolis. The accident occurred less than an hour after departure at approximately 5:40 pm CDT (2245Z). Part of the portions of the left elevator and left horizontal stabilizer on the back end of the plane were found blocks from the accident site implying an in flight break up.
It is not known at this time what caused the accident and this post will not speculate what might have contributed to the accident. However, there has been speculation throughout the pilot community that such a violent accident was due to thunderstorms or encountering a possible microburst in the area at the time of the accident. This post will demonstrate that severe or extreme convective turbulence or low level wind shear was highly unlikely at the time and location of the accident.
One pilot conjectured that there was a solid overcast several thousand feet thick. Yes, there was an overcast deck in the area as indicated by the surface observations from the ASOS on the field at KFCM. The ceiling was MVFR between 1,100 feet and 1,300 feet with visibility below the cloud deck around 10 statute miles.
KFCM 072353Z 09007KT 9SM OVC012 22/19 A2977 RMK AO2 SLP077
KFCM 072253Z 06007KT 10SM OVC013 23/19 A2976 RMK AO2 SLP075
KFCM 072153Z 08010KT 9SM OVC011 22/20 A2977 RMK AO2 SLP078
KFCM 072100Z 08009G17KT 10SM OVC011 22/19 A2978 RMK AO2
There's no way to tell the depth of this cloud deck unless there are pilot weather reports or a recent weather balloon (radiosonde) launch in the area. At 6:00 pm CDT (23Z), the NWS launches a radiosonde (weather balloon) at the Weather Forecast Office (WFO) in Minneapolis. This office is conveniently located less than five miles to the east of the accident site as shown below. So it represents an excellent resource to determine not only the cloud bases, but the actual tops and the relative stability of the atmosphere very close to the accident site. This is actually quite a rare event to have a radiosonde observation so close in time and space.
Below is the radiosonde observation (RAOB) for MPX from the 23Z launch. Couple things to note with the Skew-T log (p) Diagram. First, this is an observation, not a forecast...so the data is very accurate. There is a shallow temperature inversion just above the surface that is characteristic of a well-defined stratocumulus cloud deck. The temperature inversion and stable lapse rate above caps these clouds which limits their vertical growth. It is likely that the picture below was fairly close to what the accident pilot saw before the descent into the cloud tops.
Given the winds aloft are 10 knots or less within the cloud deck, it's unlikely the penetration of clouds during the descent would have been turbulent.
From the Skew-T log (p) Diagram, the tops of the stratocumulus deck are indicated at approximately 2,800 feet MSL with the bases at approximately 1,200 feet AGL (taking into account the elevation of where the radiosonde was launched or 942 feet). This produces an overcast stratocumulus layer that is roughly 600 feet thick (not several thousand feet thick as was mentioned by another pilot). Moreover, this thin cloud deck still allows a lot of sunlight to reach the surface. From the sounding above, you can see on the temperature profile there's a "foot" with a large lapse rate at the surface. This defines a super-adiabatic lapse rate in what is called the surface layer. That rate is typically about 6°C/1000 feet. That means the sunlight is still contributing a fair amount of heat to the surface despite the cloud deck above. Second, it would certainly not be all that "dark" under the overcast sky and would likely still produce shadows on the ground.
Also it has been mentioned that conditions were "ripe" for convection and that there were thunderstorms in the morning and in the evening with showers in the area most of the day. There was a Mesoscale Convective System (MCS) that moved through the Minneapolis area very early in the morning (around sunrise) as shown below by the morning surface observations and NEXRAD mosaic. However, by early afternoon the showers and thunderstorms moved out of the area where the accident occurred.
KFCM 081102Z 00000KT 6SM VCTS -RA BR BKN012 OVC017 21/20 A2970 RMK AO2 LTG DSNT NE AND E TSE1056 P0002 T02110200
KFCM 081053Z 00000KT 6SM -TSRA BR BKN014 OVC032 21/20 A2969 RMK AO2 LTG DSNT E RAB20 TSB17 SLP050 P0021 T02110200
KFCM 081042Z AUTO 30003KT 3SM TSRA BR FEW002 BKN014 OVC032 21/20 A2969 RMK AO2 RAB20 TSB17 P0020 T02060200
KFCM 081039Z AUTO 26003KT 1 1/2SM TSRA BR FEW002 SCT014 OVC031 21/20 A2970 RMK AO2 VIS 3/4V4 RAB20 TSB17 P0019 T02060200
KFCM 081031Z AUTO 24005KT 3/4SM +TSRA BR SCT002 SCT012 OVC017 21/20 A2970 RMK AO2 LTG DSNT SE RAB20 TSB17 P0017 T02110200
KFCM 081028Z AUTO VRB05KT 1SM +TSRA BR SCT002 OVC011 21/20 A2970 RMK AO2 LTG DSNT SE RAB20 TSB17 P0011 T02110200
KFCM 081017Z AUTO 00000KT 1 1/4SM TS BR OVC002 21/21 A2970 RMK AO2 TSB17 T02110206
KFCM 081002Z AUTO 05005KT 3/4SM VCTS BR VV002 21/20 A2968 RMK AO2 LTG DSNT SW T02060200
All of the convection moved to the ESE and by the time the flight departed in the afternoon the atmosphere was fairly stable on the direct route to Flying Cloud Airport. The RAOB above shows a huge level of surface-based convective inhibition (CINH) with a meager amount of surface-based convective available potential energy (CAPE) and a relatively high level of free convection at 12,500 feet MSL. So the conditions were definitely not "ripe" for convection at the time and location of the accident. In fact, it was just the opposite.
The only convection that persisted during the flight remained south of the Minneapolis-Saint Paul metroplex. The NEXRAD mosaic at the time of the accident shows a single dissipating cell to the southeast of the Flying Cloud Airport which is over 30 miles from the accident site.
Below is a loop of the Level II NEXRAD base reflectivity from MPX from 20Z to 23Z. The center of the ground clutter around the radar site shows the WSR-88D blind spot over the radar as a small circle. The accident was located less than five miles to the west of this location. Most of the stronger cells stayed well south of the Flying Cloud Airport during this time. Therefore, it is unlikely the accident aircraft experienced any convective turbulence during a descent into Flying Cloud Airport. In fact, the clutter region around the radar site actually "blooms" a bit at the end of the loop implying the atmosphere was stabilizing even more into the early evening.
The Infrared (IR) satellite image at the time of the accident clearly shows the MCS that plagued the Minneapolis area in the morning and moved to the east throughout the day. There was a stratocumulus deck that prevailed over the accident site.
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Dr. Scott Dennstaedt
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CFI & former NWS meteorologist
