Need insight on 1975 airplane crash

[cdillard]

Hello, pprune.org. I have a question that involves the plane crash of a friend 35 years ago in Butler, Tennessee. Here is a transcript from FAA of the crash:

"3 KILLED BUTLER TN 1975-12-25, AIRFRAME FAILURE-IN FLIGHT; PILOT INDUCED; MTN RIDGE IN AREA OF ACDNT APRX 5700FT MSL.WINGS,TAIL SECTION SEPARATED PRIOR TO IMPACT."

Can someone provide a scenario/insight of what "AIRFRAME FAILURE-IN FLIGHT" means? Also, the relevance of "MSL.WINGS,TAIL SECTION SEPARATED PRIOR TO IMPACT."

The pilot was approaching an airport in the eastern Tennessee mountains and crashed.

Thanks for any insight you can provide, pprune.org.

[411A]

What type airplane?

[Airclues]

The pilot flew into challenging weather conditions. All aircraft have structural limits. If the actual levels of 'g' exceed the design limits then the airframe can break up. It would seem, in this case, that the pilot lost control of the aircraft and exceeded the limits.

IAD76AI044 (http://www.ntsb.gov/ntsb/brief.asp?ev_id=51399&key=0)

Dave

[411A]

MTN RIDGE IN AREA OF ACDNT APRX 5700FT MSL.WINGS,TAIL SECTION SEPD PRIOR TO IMPACT.

Hmmm, perhaps the pilot found a rotor on the lee-side of a mountain range.

[Mad (Flt) Scientist]

Can someone provide a scenario/insight of what "AIRFRAME FAILURE-IN FLIGHT" means? Also, the relevance of "MSL.WINGS,TAIL SECTION SEPARATED PRIOR TO IMPACT."

The first one, Airclues basically answered. In case you're unfamiliar with the 'g' terminology, the forces on an aircraft (and other things too) can be expressed in terms of the resulting acceleration that would be caused. The units used are often 'g', where that means "the acceleration due to normal earth gravity". As a result, a 2'g' condition is accelerating twice as fast as gravity would cause, and the force being applied is twice the weight of the object.

For the second part, there's a missing space. It should be:
"MTN RIDGE IN AREA OF ACDNT APRX 5700FT MSL. WINGS,TAIL SECTION SEPARATED PRIOR TO IMPACT."
which means
"Mountain range in area of accident approximately 5700ft above Mean Sea Level. Wings and tail section separated (from fuselage) prior to impact." (The latter being a symptom of the fact that the structure failed "in-flight".)

[eckhard]

The probable sequence of events is given in the NTSB report:

PROBABLE CAUSE(S)
PILOT IN COMMAND - CONTINUED VFR FLIGHT INTO ADVERSE WEATHER CONDITIONS
PILOT IN COMMAND - SPATIAL DISORIENTATION
PILOT IN COMMAND - EXCEEDED DESIGNED STRESS LIMITS OF AIRCRAFT

The 66-year-old pilot was not instrument rated. He probably entered cloud inadvertently. The aircraft then entered a spiral dive. The pilot saw the increasing airspeed and reducing altitude and reacted by pulling back on the control wheel. With the wings still at an angle of bank, this action would increase g without reducing the descent. The pilot pulls harder - eventually the structural g limits and maybe the speed limits are exceeded and the (wooden) wings fail.

Unfortunately this scenario is repeated several times a year around the world. Most light aircraft are marginally unstable in the rolling plane and will enter a spiral dive without too much encouragement. The ways to stop this happening include:

1. Stay out of cloud unless you are instrument rated;
2. If you're not instrument rated, get regular practice in conducting a 180 degree level turn under the hood;
3. Learn and then practice regularly how to recognise a spiral dive and how to recover from one.

[Northbeach]

I will try and answer the questions you asked, but understand that I have no knowledge of the specific accident you referenced on Christmas day back in 1975.

“Airframe” means the physical body of the aircraft to also include the cabin where the pilot and passengers would sit, the wings and tail.

“Airframe failure-in flight” would mean that the airplane was pulled apart by forces in flight rather than being destroyed by the impact with the ground. In other words the airplane was broken up in various pieces [in this case the wings and tail broke away from the cockpit & cabin section in the air] before the wreckage struck the earth and was further damaged.

When airplanes are designed and built they are stressed to withstand a certain amount of additional G-forces [force of gravity]. This airplane was subjected to forces, during its final and accident flight, that exceeded the design limitations and/or the airplanes ability to withstand stress.

However if the airplane was previously damaged, intentionally abused, had some sub-standard repair job or was sabotaged; being compromised in such a manner could also lead to "airframe failure in flight".

“Pilot induced,” means that in the opinion of the writer of this accident summary the pilot of the accident aircraft flew his/her aircraft in such a manner to eventually lead to the destruction of their airplane due to the airplane being subjected to forces beyond its capacity. This can be due to pilot incapacitation or the pilot lost control of the airplane and was unable to recover control and return to safe flight.

"Pilot induced" being in contrast to there being some design fault of the airplane or outside element that caused the pilot to be unable to react in some way to avoid the accident. The pilot was flying the airplane and under the pilot's controll the aircraft was destroyed [was not hi-jacked, shot down nor had a mid air crash with another airplane in flight].

Think of it in this way; baseball bats are tough. They are designed to smack baseballs and send them flying. However if you take a wooden baseball bat to a steel beam the bat is going to break. The bat will break because it was designed to hit baseballs not steel. Airplanes are tough by design, but if you take them beyond their design limits bad things happen.

If I were to summarize the sentences you highlighted in non-aviation language my summary would sound something like this: the pilot of the airplane lost control during flight and the airplane broke apart in the air due to it being subjected to forces beyond its physical limits. The wings and tail of the airplane separated from the rest of the airplane prior to the accident airplane hitting the ground.

I am in no way passing judgment on the pilot of the accident flight you asked about, or the type of airplane nor am I offering any kind of explanation as to why the loss of life occurred – I simply do not know nor do I have any of the facts. With those limitations I hope you find some use in my explanation.

[barit1]

Another accident in the same general region was this (http://www.ntsb.gov/ntsb/brief.asp?ev_id=20060501X00494&key=1). The overloads in this case were the result of the pilot inadvertently entering severe weather with gusts that overloaded the airframe and caused it to fail.

[barit1]

eckhard:The pilot saw the increasing airspeed and reducing altitude and reacted by pulling back on the control wheel. With the wings still at an angle of bank, this action would increase g without reducing the descent. The pilot pulls harder - eventually the structural g limits and maybe the speed limits are exceeded and the (wooden) wings fail.


You seem to imply that wood wing construction is inferior to others (aluminum, composite, ...) and I beg to differ.

While wood wings, tail etc. need proper care and inspection (as do other materials) I submit that in general it's no less safe than others. One can easily find many Bonanza, Comanche, etc. accident reports in which the aircraft came apart in the air under similar circumstances.

And one wood-wing type in particular - the Howard DGA (550 built before/during WWII) - has no record of such failure. Its Vne is 235kt.

[eckhard]

Fair comment, barit1.

Would that I had your knowledge!

Merry Christmas!

[barit1]

An example of a spatial disorientation accident (i.e. the pilot continues flight into cloud or fog, loses sight of the natural horizon, and becomes forced to rely on instruments to maintain correct attitude) is this one near Martha's Vineyard (http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20001212X19354&ntsbno=NYC99MA178&akey=1).

This report is unusually detailed because of the high public profile of the pilot, and the public scrutiny of the investigation. Thus it contains much detail on the nature of spatial disorientation. Unfortunately this sort of accident is not uncommon among amateur pilots. In this particular case he became disoriented at low altitude, and struck the water before the speed increased to the point of in-flight disintegration.