Cessna C208B down near Seattle, 4 deceased

[atcclears]

Update from a local news article:


The crew of the Cessna 208B had already done three days of test flights, but the day before the crash they ended early because one of the crew members felt ill. The crew went back up the following day and was testing the Cessna’s aft center-of-gravity stall characteristics when the plane crashed, the agency said.

Witnesses said the airplane broke up in flight and descended in a near-vertical corkscrew to the ground and several witnesses reported seeing a white plume of smoke as the airplane broke into pieces, the NTSB report said. The agency has previously said a wing broke away from the plane during the crash.

[Pilot DAR]

and was testing the Cessna’s aft center-of-gravity stall characteristics

Two thoughts come to my mind with that information: If there had been a ballasting error, and the C of G was actually further aft than intended, recovery could have been an extra challenge. If the stall was allowed to develop into a spin (which may be inferred from the final very tight turn in the ADS-B trace), recovery would be demanding. If both factors were combined, it would be a great challenge for the pilots to recover.

I am responsible for not detecting a couple of ballasting errors during flight testing over the years, which has made me extra vigilant in that regard. I errantly accepted a "behind the aft limit" Cessna 185 for spin testing once, and it was immediately apparent that spin recovery was noticeably more difficult behind the aft C of G limit. Lesson learned!

[Pilot DAR]

Summary article in Flying Magazine here:

https://www.flyingmag.com/ntsb-relea...that-killed-4/

[212man]

It is very sad for the crew, the last part of that ride was terrifying!

I’m sure, although possibly less so than one might imagine. I was involved in an airprox between two helicopters, whilst acting as an examiner/observer on an IF training session in one of them. I saw the other aircraft suddenly appear from behind the coaming in our 1 o’clock and slightly low. We were in his 7 o’clock, and converging, so totally out of view as he was single pilot. I started saying loudly “turn left, turn left” but there was no reaction. RHS pilot was fixed on his instruments and couldn’t understand why I was so upset about a 3 degree ADF divergence, LHS pilot I don’t know. In the space of about 5-6 seconds I went from calm, to surprise, to shock, to fear then to total relaxed acceptance of imminent death. Then the LHS pilot turned us left and we missed - my report said 20ft vertical and 50ft horizontal separation. The subsequent physiological reaction was intense! Point being, when you know you are actually about to die, it’s not as terrifying as you would expect!

[spornrad]

Originally Posted by Pilot DAR;11334283...The ADS-B track can be found here:

[url

https://globe.adsbexchange.com/?icao=a1abe2&lat=47.899&lon=-122.047&zoom=14.6&showTrace=2022-11-18&timestamp=1668795559[/url]

I note, with alarm, a "groundspeed" of 176 kts (one knot faster than Vmo),.. !

In that dive the ADS-B GS reaches 197 kt in the dive. So the airspeed was probably faster and beyond Vne.

[Pilot DAR]

In that dive the ADS-B GS reaches 197 kt in the dive. So the airspeed was probably faster and beyond Vne.

And this is the part of the equation which is alluding me a little... The airplane moved forward around 3000 feet while losing around 7000 feet of altitude, so more down than forward. If the "forward" is the base of the triangle, described as "groundspeed", and peaked at 197 kts, and the "down" is the altitude lost, the motion along the hypotenuse of the triangle would represent the actual airpseed?

It is very sad for the crew, the last part of that ride was terrifying!
I’m sure, although possibly less so than one might imagine.

Yes terrifying. Very sadly, there will be no memories of the crew. That said, having experienced my own terrifying experience when a training flight went wrong, I can say that I have very little recollection of it at all. Only enough, to know that yes, it did happen to us. That, and another extremely traumatic experience a couple of years back, which I know I witnessed in its full horrifying duration but do not recall at all, have me convinced that the mind simply turns off observation when things get really really bad. I hope this for the crew, as I would for any person experiencing sheer terror, with zero opportunity to mitigate it. The movies show us an increasingly terrified pilot hurtling toward earth and a certain death. That's the movies, I think (hope, anyways) that our actual life experience has some kind of built in protection for our emotions.

[megan]

I can say that I have very little recollection of it at all. Only enough, to know that yes, it did happen to us

Take off from an oil rig, 125'ASL at rotation, massive explosion and glass screen of engine instrumentation turned seemingly all red, memory ceased at that point, memory regained on climb out when other chap asked "do you want me to cancel the audio tone", not aware of it until he asked, nor aware that he had put out a mayday during the dive for the water to gain airspeed.

[RatherBeFlying]

Using two significant figures and 3,600 seconds per hour:

3000' ≈ .49 nm
7000' ≈ 1.2 nm
GS ≈ 200 kt

Time in seconds over horizontal 3000' ≈ 0.49 / 200 x 3600 ≈ 8.8

Speed over vertical 7000' ≈ 1.2 / 8.8 x 3600 ≈ 490

Speed down slope ≈ (200^2 + 490^2)^½ ≈ 530 kt

The structural failure may have happened before achieving this speed.
​​​​​​

[Pilot DAR]

Thanks RBF, I did my modest math, and got very similar results. I figured I must have made an error along the way, and set my results aside. Like a few other planes I've dived, the Caravan will build up speed very quickly when pointed down (as opposed to a few floatplanes I've dived, when once pointed down, I had to add power to achieve the required speed).

The two photos in the preliminary NTBS report do show the airplane descending vertically, probably already broken up, and they can probably validate the speed with those photos.

[First_Principal]

Not quite the same but in the Pacific GA forum TWT has posted this report which discusses what appears to be structural failure of a 210 wing in flight...

[Pilot DAR]

The report of this accident can be found here:

https://data.ntsb.gov/carol-repgen/a...ort/106317/pdf

This is worth the read, both from the perspective of flight test discipline, pilot familiarity with aircraft avionics systems, and the factor of the Garmin Electronic Stability and Protection (ESP) system supplementing control of the airplane while the unaware pilot was stall testing, and recovering. Exactly this has also happened to me during recent flight testing, deliberately stalling the airplane, unaware that this system was operating in the background I managed to not overspeed the plane as I recovered, but it was a messy recovery, while I figured out that my test subject airplane also had this system. I'll be reviewing this report with great interest, but the "background" ESP system is a factor which cannot be overlooked here. The airplane I've been testing and will approve (other modifications) is going to have an approval with warnings about the ESP system operation.

Complex automation concerns me a little, with the need for more pilot familiarity. A background system which the pilot may not even know is there, supplementing control of the airplane without the pilot knowing, really worries me!

[BFSGrad]

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The pilot’s improper recovery following a departure from controlled flight after an intentional aerodynamic stall, which resulted in an exceedance of airspeed limitations, airframe overstress, and a subsequent inflight breakup.

NTSB Aviation Investigation Final Report - N2069B

[Pilot DAR]

Yes, that is the state "cause" of the accident, but that's the "simple" cause. There are factors, and in this accident, the factors are big. This is not simply an accident where two very experienced pilots mismanaged an intentional stall, the pilot was "unfamiliar" (page 2, first para) with the ESP system (which DID affect the airplane's handling characteristics, and which is not "presented" as being there, nor in operation), and there was "confusion" about elevator forces and position during intentional stalls (page 19, first para).

The very same thing happened to me stall testing a 182 amphibian last month, where similarly, I was entirely unaware that the airplane was equipped with ESP, nor how it worked and what its control inputs would be. My first awareness of the presence of a supplemental control system affecting my flying was an aural annunciation of "Autopilot engaged" (when I have not engaged it at all - it turned itself on), and pitch trim wheel moving at three times the normal electric trim speed (I later timed it). But, at that moment, the ESP system had already been applying control forces - with zero pilot indication whatever. All of this would be like the person in right seat pushing and pulling the control wheel while, and as the PF, you did not notice that they were doing this. If you were aware, you'd tell them to get their hands off the controls.

And, during flight testing for stall handling, the pilot is thinking about many other things about the test objective and observations, and the last thing he expects will be interference with the controls. I've flown hundreds of flight test stalls in Caravans, and I can say that the Caravan is one of the most benign stall handling airplanes I have ever flown. And, I have spun Caravans many times. They are appropriately tolerant of mishandling right up to the spin entry, but, once in the turn/roll, recovery is very demanding. I opine that this pilot was not prepared to recovery from an unintended spin. In fairness to him, from his previous stalling experience on the Caravan, he probably did not expect to enter a spin. So, if not intending to enter a spin, and obviously having had some practice spinning them, what tipped the balance in this event from stall to spin? Unfamiliar with a [hidden] system, and confusion when it actuated. I admit to also being unfamiliar when I test flew the 182. Where is the point where pilots with decades and thousands of hours on type need to start asking if this airplane is the same to fly as all those they have flown before, when it should be?

Based upon my startling experience with this same system in the 182, and after considering this report, I feel strongly that more is needed to assure that pilots are aware of such systems. The world vigorously criticized Boeing for MCAS system, I don't see this as being all that different when, again, the airplane is being flown by pilots unfamiliar with the system.

This accident, and the report are opening more questions for me than answering them....

[hatstp]

This highlights one of those significant challenges with flight testing: 'know your test article'. The challenge comes when aircraft that are used for STC type work are a 'Frankenstein' of mod state/STCs etc and the ability to fully understand the baseline standard and how that might affect testing, test outcome, mitigation etc is complex and challenging.

The other valuable lesson for all flight test professionals is the ability to be critical of unexpected results; to be able to pause, analyse and conclude why something didn't go as planned (failed). This also then needs any mitigation, risk reduction measures etc to be assessed for continued validity. Some similarities here with the G650 crash in 2011; unexpected results from previous testing (wing rock) that were not fully 'understood' and may have allowed a pause on testing to be instigated whilst a full analysis was carried out.

[fdr]

Originally Posted by Pilot DAR

This accident, and the report are opening more questions for me than answering them....

I tend to agree P-DAR. As sad as a number of the findings are to read, I think that a number of them seem to be overstated in importance, and have not been conjugated for what was expected to occur.

Random musings:

• The modification to the gear is not relevant to a stall, overspeed/overstress outcome, other than increasing drag and being beneficial in reducing the speed buildup in the dive.
• The addition of stall fences on the wing themselves would not contribute appreciably to an aggravated stall roll off directly.
• Indirectly, addition of stall fences may increase rolling moment due to greater CL inboard in the propeller wake field. That still appears improbable, the fences are well outboard at part span, and the effect inboard would be minimal, but it is possible.
• The addition of gurney tabs inboard on the TE flaps can be a factor in increasing roll-off at the stall, whether flaps up or down. At any occasion that gurney flaps are added to the inboard TE area, the change in the CL/aoa needs to be accounted for. In simple terms, the gurney flap moves the CL/aoa slope by around 4 degrees +/- dependent on its h/c. The TE tab also alters the aoa that peak CL occurs, and that can be altered by design to end up with a very benign stall, by effectively increasing wash-out. OTOH, an aggressive TE tab with added VGs on the flap will result in a delayed stall inboard, and that results in an aggressive stall behavior.
• The C208B by memory has a set of ogive planform VGs on the flap at around 10%fc, and that would aggravate the stall roll-off for a straight addition of a TE tab to the flap.
• A constellation of low profile VGs outboard would result in a return to a conventional roll-off if the flap TE is modified. My own flight testing in this area used ramp, vane and similar VGs the latter in co-rotation-inboard, outboard and couter rotating geometry. I prefer counter rotating but that is not a large change in effectiveness, other than where yaw is involved in which case counter rotating is a much better proposition.
• Stall fences work well on swept wings, particularly to maintain aileron authority at moderate to high aoa. with a prop, and a straight wing, it wouldn't be my first choice for mitigation of the TE tab effect. (James Raisbeck unfortunately departed this earth well before his use by date, and that is a shame, he would have been familiar with what I am indicating here, as would Bob Desroche, from micro, who is nowadays feeding his horses and generally done with aerospace. Other than these guys, John C. Lin from NASA would be a good resource for anyone to talk to on this issue which is probably outside of the wheelhouse of the NTSB).
• Would like to see what the C208B roll off is with a normal un-accelerated turning stall. I have a suspicion that it will be a fair rate to the left. If I was the NTSB investigator, I would have asked for some background from Cessna for that. I am not interested in blame, I would think that it is worth knowing if a latent risk exists in the aircraft other than the configuration that the guys got into.

Back in 1991-97 I ran a series of tests on TE tabs on flaps on a long suffering PA23-250D that I had. The testing included a number of configurations, including flap TE, cove tabs (really cool) flaps with VGs, flaps with VGs and TE tabs, and outboard wings with VGs to offset the change in CL/aoa inboard. The effect of TE tabs is spectacular. To verify the effectiveness we deliberately set up for aggravated stall conditions, which have a validation of the effective change achieved by the various VGs, as well as the TE tab. The reference case stall speed of 52KIAS was reduced to 39KIAS, and that had to be calculated from GPS, the IAS was inaccurate at the very low speed case. The drag change was also notable, any TE tab on a flap will result in a lower drag at the full flap position, but when applied in conjunction with a transverse tab in the flap cove, the effect is spectacular. In tuft visualisation, fully attached flow exists over the flap upper surface, at F50, and at Vs for the associated stall speed. The flap cove tab results in a condition of off body recirculation, which gives a jet flow over the upper surface which is pretty cool to observe. The CFD shows that clearly, the tufting verifies that the CFD was not an artefact.

Originally Posted by hatstp

This highlights one of those significant challenges with flight testing: 'know your test article'. The challenge comes when aircraft that are used for STC type work are a 'Frankenstein' of mod state/STCs etc and the ability to fully understand the baseline standard and how that might affect testing, test outcome, mitigation etc is complex and challenging.

The other valuable lesson for all flight test professionals is the ability to be critical of unexpected results; to be able to pause, analyse and conclude why something didn't go as planned (failed). This also then needs any mitigation, risk reduction measures etc to be assessed for continued validity. Some similarities here with the G650 crash in 2011; unexpected results from previous testing (wing rock) that were not fully 'understood' and may have allowed a pause on testing to be instigated whilst a full analysis was carried out.

Yup.

Doing a rapid pitch rate rotate, at aft CG is an interesting test point; doing that on a wing that is swept, and has no LE device, is more than interesting. I understand why smart people at Gulftream have avoided having a LE Device on their wings, but I would prefer not to fly them due to the inherent sensitivity to contamination, and to non linear CL, and CM associated with LESB's. Wing rockin' should be a hint that things might be less than neat.

Originally Posted by BFSGrad

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The pilot’s improper recovery following a departure from controlled flight after an intentional aerodynamic stall, which resulted in an exceedance of airspeed limitations, airframe overstress, and a subsequent inflight breakup.

NTSB Aviation Investigation Final Report - N2069B

thank you and the mod for the link correction. I would thank you but that would seem to be for the wrong reason. I am not sure I agree with the NTSB conclusions on this one.