Updated: May 4
On the evening of January 8, 2022, the pilot and other occupant on a Beechcraft 58 Baron were fatally injured in an accident shortly after departing out of the Spirit of St Louis Airport (KSUS). The preliminary NTSB report can be found here. According to Flightaware, the flight departed at 7:10 pm CST headed to Centennial Airport (KAPA) in Denver, Colorado. There has been much speculation and debate that airframe icing may have played a role in the accident. The analysis to follow along wtih this video will confirm that airframe icing was not a factor in this accident.
Please note that this discussion is for educational and entertainment purposes only and may contain errors and omissions. If you like what you read here, please visit https://ezwxbrief.com for the best source of online weather and education. If you are looking to learn more about aviation weather, please considering purchasing my book, Pilot Weather: From Solo to the Airlines.
According to the Flightaware log, the pilot climbed to the cruising altitude of 8,000 feet MSL. Shortly after, the plane appears to have departed controlled flight during a rapid descent and subsequent impact with terrain 2.5 mi south of New Melle, Missouri (12 miles west KSUS). A special observation was taken at 0107Z for KSUS that reported southerly winds, two statute miles surface visibility in mist and an overcast ceiling at 1,000 feet. The ceiling was variable from 800 feet to 1,300 feet and no precipitation was reported. The temperature at the surface was +7°C with a dewpoint temperature of +6°C.
KSUS 090107Z 19007KT 2SM BR OVC010 07/06 A2996 RMK AO2 CIG 008V013
It is common for a pilot to suspect airframe icing in the clouds aloft given a surface temperature of +7°C. Many pilots are incorrectly taught to use the standard lapse rate of 2°C/1,000 feet to calculate the freezing level. On any given day, the lapse rate of the troposphere is rarely standard. The KSUS airport elevation is 463 feet and using a standard lapse rate the altitude of the lowest freezing level should be ~4,000 feet.
1,463 ft --> +5°C
2,463 ft --> +3°C
3,463 ft --> +1°C
4,463 ft --> -1°C
On this particular evening using the standard lapse rate in this way would lead to an incorrect freezing level. This is echoed in the freezing level forecast below which suggests that the freezing level west of St. Louis was between 11,000 feet and 13,000 feet. That would create an error of 8,000 feet if the standard lapse rate is used.
It is of utmost importance to understand that the standard lapse rate should never be used in this fashion to make decisions about the temperature aloft. It seems that way too many pilots are opting to do this which is extremely dangerous, especially when the actual environmental lapse rate is greater than the standard. In fact, some popular YouTube personalities and their followers are also pushing this kind of thinking. Instead, why use a standard lapse rate when you can use the actual/forecast temperatures aloft to make decisions?
The only time the standard lapse rate should be evaluated is for any departure from standard calculations used in the various POH/AFM performance tables. This should be made very clear during a pilot's primary training, but evidently too many flight instructors are still teaching this to pilots. Using the standard lapse rate to determine the lowest freezing level is akin to expecting the gasoline prices in the Los Angeles region to be similar to the national average. When you arrive in Los Angeles you will be sorely disappointed to find the gas prices to be anywhere near the average.
First, let's take a look at the big weather picture. Shown above is the surface analysis chart valid at 0000Z on January 9th. The approximate location of the accident is marked in the center of the picture above. There was a warm frontal system that was moving from south to north at the time of the accident. Surface winds in the area were generally out the south. This effectively means that warm air from the south was overrunning cold air to the north. This likely implies a strong surface-based temperature inversion. Given the surface temperatures are well above freezing, this likely means a fairly high freezing level.
This is confirmed above by examining the 850 mb temperature (~5,000 ft MSL) from the GFS model 0000Z analysis above. Around the St Louis area, the temperature is greater than +9°C at 850 mb. The Current Icing Product (CIP) calibrated icing probability analysis below also depicted zero risk of airframe icing at 8,000 feet MSL.
Furthermore, the icing probability composite (a composite of all altitudes from the surface to FL300) shows no risk of icing in the St. Louis area. The little area of icing on the composite shown just to the east of St. Louis occurred between 12,000 and 14,000 feet.
At the time of the departure from KSUS, there was a solid line of deep, moist convection meeting SIGMET criteria to the east and southeast that was moving east and away from the area. This was not a factor for the flight.
At the time of the accident, no precipitation was being reported. The Level II radar data from the St. Louis/St. Charles NEXRAD site below only showed some very light precipitation or perhaps drizzle to the north of the accident site (blue dot). This is the 0.5° elevation scan. The radar site is in the middle of the image and the other returns around the radar are non-precipitation returns called ground clutter. This was likely associated with the warm front moving to the north through this area.
There isn't a radiosonde launched near this location, but there is a radiosonde released near Lincoln, Illinois valid at 0000Z. The actual launch is 2300Z, but it's likely the best observation available to determine the temperature profile aloft. Note that this site is about 100 nm northeast of the accident site, but as will be shown below, has a very similar environment. This shows the classic deep surface-based inversion with the freezing level approximately 9,000 feet MSL.
The RAOB above does show the temperature at the surface to be slightly below freezing giving rise to a freezing rain event in this area as can be seen by the METAR for the Abraham Lincoln Capital Airport (KSPI) near the time of the balloon's release. However, this was much further north in the colder air.
KSPI 082314Z 18014KT 2SM -FZRA BR OVC005 01/01 A2997 RMK AO2
The color-enhanced IR satellite above valid around the time of the accident shows a very homogenous area of warm-topped clouds with tops showing about +2°C. This also shows the very cold convective cloud tops to the southeast. With a very homogeneous cloud coverage, it's likely the weather near Lincoln and the west of St. Louis was very similar giving rise to a similar temperature profile, albeit warmer near the accident site.
As shown below there were no pilot weather reports of icing in the region at the time of the accident.
Furthermore, there were no advisories for airframe icing valid at 0000Z to include SIGMETs (not shown), G-AIRMETs (shown below) and CWAs (not shown). If there was a serious icing risk, there would have been advisories present.
Moreover, some pilots online have been speculating that somehow even if the air temperature was above freezing that "Bernoulli's principle" played some role in lowering the temperature of the air at the wing's boundary layer creating a way for ice to accrete on the airframe even at +6°C (the temperature at 8,000 feet at the accident site). If this were true, then there would be hundreds if not thousands of icing accidents occurring each year. In fact, it's just the opposite. Most aircraft do not start to seriously accrete ice until the temperature is several degrees below freezing.
This is due to a well known effect called kinetic heating that actually increases the air temperature of the boundary layer of the leading edges. This is primarily through adiabatic compression of the air on the immediate leading edges and is typically referred to as ram air temperature rise (also called the total air temperature). The amount of temperature rise is proportional to the aircraft's true airspeed. In fact, many aircraft with a certified ice protection system actually senses the total air temperature (TAT) and then computes the outside air temperature (OAT) based on the true airspeed.
Given the analysis above, it's reasonably clear that airframe icing was not the root cause of this accident. The pilot likely climbed from the surface to cruise altitude in IMC, but that climb was at a temperature well above temperatures conducive to airframe ice.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist