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In the NTSB Powerplant Group Chairman Factual Report for the 2014 crash of the King Air B200 in Wichita Kansas (NTSB# CEN15FA034), I read:

"Neither propeller was feathered before impact. Both engines exhibited multiple internal damage signatures consistent with engine operation at impact. Engine performance calculations using the preimpact propeller blade angles (derived from witness marks on the preload plates) and sound spectrum analysis revealed that the left engine was likely producing low to moderate power and that the right engine was likely producing moderate to high power when the airplane struck the building."

What are witness marks on a propeller blade? And how do they provide data on estimated propeller blade angles at the time of impact?

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  • $\begingroup$ Would you mind telling us which NTSB report you're quoting? There're kind of a lot. $\endgroup$
    – Vikki
    Jun 8, 2021 at 18:23
  • $\begingroup$ @Vikki-formerlySean, I have updated the post with the information. Thanks! $\endgroup$
    – BtureP
    Jun 8, 2021 at 18:31
  • $\begingroup$ You're very welcome! :-) $\endgroup$
    – Vikki
    Jun 8, 2021 at 18:34
  • $\begingroup$ One other thing - you said you were reading the Powerplant Group Chairman's factual report, but the quoted passage is from the final accident report. $\endgroup$
    – Vikki
    Jul 21, 2021 at 16:31
  • $\begingroup$ The wording in the powerplant report: "No evidence of pre-impact failure was found. Evaluation of the propeller damage determined that neither propeller was feathered at impact. Witness marks provided data from which estimated propeller blade angles at the time of impact could be derived. The marks indicated that the left powerplant was producing low to moderate power and the right propeller was producing moderate power at the time of impact." $\endgroup$
    – Vikki
    Jul 21, 2021 at 16:34

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Note that the report says "derived from witness marks on the preload plates", so the investigators did not use witness marks on propeller blades to determine the blade angle. This might have been possible, as scratches and the deformation of the blade tip do, to some extent, manifest blade angle, but you would need to take into account the speed at which the plane was travelling at the moment of the propeller strike. There is also the problem that propellers tend to get quite mangled in accidents such as this, so the endresult may be useless as evidence.

Now, the preload plate is more suitable for determining the angle of blade at the time of impact. It is a part at the root of the propeller, inside the propeller hub: Propeller-Harzell As the propeller hits the ground (or any solid(ish) object) the force of the impact is relayed all the way to the bottom of the blade, where the preload plate sits. This force will leave an indentation, a witness mark on the preload plate, and from this mark the investigators can determine the angle of the blade at the time of the impact, as the place of the mark will be specific to the blade angle.

Since it was briefly discussed in comments: A witness mark can be both a deliberate marking on a part, made to ensure proper installation for example, but it also means a mark induced by force during an accident or some other event:

Waywordradio.org: witness mark (thanks Chris)

In good old days one common witness mark the accident investigators were searching for, were the small scratches the gauge needle made on the dial during an impact. These witness marks pretty accurately showed what each gauge was reading at the moment the plane crashed if the impact was at a favourable angle.

Picture source: Propeller Owner's Manual - Harzell Propeller Inc.

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    $\begingroup$ I'm only going by that cutaway drawing and some assumptions about the mechanism, but it looks like by the time the force is transmitted to the preload plate, analysing the damage will be far simpler, a matter of a mark at an angle, rather than trying to decipher far worse damage $\endgroup$
    – Chris H
    Jun 7, 2021 at 8:25
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    $\begingroup$ My understanding, from a manufacturing perspective, that witness marks are made in machined parts ahead of time and are used for assembly Quality Control, is there any reference you can find in terms of use of witness mark as an impact feature? $\endgroup$
    – crasic
    Jun 7, 2021 at 17:25
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    $\begingroup$ @crasic Here is a reference: waywordradio.org/witness_mark "generally, an intentional, accidental, or naturally occurring spot, line, groove, or other contrasting area that serves as an indicator of certain facts" ... "in construction and manufacturing, a line, groove, score, notch, cut, or written indicator made on the surface of material to impart information, such as where to cut or join; in forensic investigation, a surface groove, smear, stain, abrasion or other feature that can serve as evidence." $\endgroup$
    – Chris
    Jun 7, 2021 at 17:56
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    $\begingroup$ @Jpe61, your example image is almost spot on. The NTSB report concerns a HC-E4N-3. $\endgroup$
    – BtureP
    Jun 7, 2021 at 18:24
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    $\begingroup$ @BtureP I just picked a random one from the manual 😃 $\endgroup$
    – Jpe61
    Jun 7, 2021 at 19:22

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