Just what is a Crash, anyway?

Discussion in 'Hangar Flying' started by Aerowerx, Nov 1, 2019.

Help Support HomeBuiltAirplanes Forum by donating:

  1. Nov 1, 2019 #1

    Aerowerx

    Aerowerx

    Aerowerx

    Well-Known Member

    Joined:
    Dec 1, 2011
    Messages:
    4,986
    Likes Received:
    1,357
    Location:
    Marion, Ohio
    There have been a number of discussions recently, on the crash survivability of different designs and construction methods. And also some mention of future readers getting the wrong ideas from reading the threads.

    I think "what is a crash" should be defined along with whether or not it is survivable.

    Lets say you are in free fall from 8000 feet AGL. Will the occupants survive, regardless of the type of construction?

    What about an engine out, maintain control, perfect approach, but your gear catches a rabbit hole and collapses?

    Are both of these scenarios "crashes"? Are your chances of survival the same in each?

    And there are an infinite number of possibilities in between these two. Not to mention the extent of injuries classified as "surviving" (walk away with no bruises? 2 years in rehab learning to walk? a veggie for life?)

    So just what is meant when someone says "material X with construction method A is more survivable than material Y and construction method B"?
     
  2. Nov 1, 2019 #2

    Vigilant1

    Vigilant1

    Vigilant1

    Well-Known Member Lifetime Supporter

    Joined:
    Jan 24, 2011
    Messages:
    4,053
    Likes Received:
    1,796
    Location:
    US
    I'm not sure that having a big confab in arguing about what a "crash" is will change anyone's mind or lead to new knowledge.
    My objective is to give aircraft occupants an improved chance of avoiding/reducing injury when they aviate. Obviously, like all other design objectives and criteria, there are tradeoffs. That can be in weight (and therefore ROC, etc), money, build complexity, stall speed, etc. But you can't deliberately make the tradeoff decision if you don't know the options.
    IMO, too often folks just say "well, this hobby has risks" instead of digging a bit deeper and making some deliberate choices that affect chances of survival in a crash/forced landing/accident/incident.
    An example: We know that providing about 4" of well-designed "stroke" space under the spinal column of aircraft occupants can significantly reduce the chances of spinal injury in even a significant "flat" impact with accompanying high sink rate. Chances of avoiding spinal injury improve even more if this is accompanied by other measures to reduce vertical acceleration experienced by the occupants (landing gear design, other crush zones, etc). This is a low cost, high payoff modification that reduces the risk of injury from a common occurrence (dropped-in landing, etc). We don't need to define a "crash" to decide whether making these types of modifications are a good idea (in each case).
     
    Last edited: Nov 1, 2019
  3. Nov 1, 2019 #3

    Hot Wings

    Hot Wings

    Hot Wings

    Well-Known Member HBA Supporter

    Joined:
    Nov 14, 2009
    Messages:
    6,382
    Likes Received:
    2,306
    Location:
    Rocky Mountains
    When I think of protection for a pilot I think in terms of 1.3 Vs and having to land in rough ground/trees. Anything more than this puts us into BRS thinking mode. While it may be possible to build to Formula One standards it just isn't practical for HBA style aircraft.
     
  4. Nov 1, 2019 #4

    Topaz

    Topaz

    Topaz

    Super Moderator Staff Member

    Joined:
    Jul 30, 2005
    Messages:
    13,732
    Likes Received:
    5,411
    Location:
    Orange County, California
    fly2kads, Vigilant1 and bmcj like this.
  5. Nov 1, 2019 #5

    Vigilant1

    Vigilant1

    Vigilant1

    Well-Known Member Lifetime Supporter

    Joined:
    Jan 24, 2011
    Messages:
    4,053
    Likes Received:
    1,796
    Location:
    US
    1.3Vs forward velocity into an unmoving solid object (e,g, tree trunk, rock, structure, etc) would cover most forced landing scenarios.

    As far as vertical velocity (well, normal to the floor), what would you set as an upper bound for consideration? 25 FPS (17 MPH, 1500 FPM) sounds like a lot, but it would cover the "mush" rate of descent of many aircraft and equates to what we see after a vertical drop of 10 feet. With >perfectly consistent< (unachievable) deceleration (including landing gear, seat pan, and any cushions under the occupant) at 20 G, 6" is enough to come to a complete stop from that speed.

    Edited to add: The ref provided by Topaz (see page 3-3, Table 3-2) indicates the FAA's standards for "Test 1" (primarily a vertical impact scenario) start with a vertical impact of 31 FPS and require a peak deceleration of no more than 19Gs for the front seat occupants. Their standards for "Test 2" (primarily a horizontal impact) start with a speed of 42 FPS (just 29 mph :eek: ) and require front seat occupants to experience peak deceleration of no greater than 26 Gs.

    See 3-6 (table 3-4 ) for other recommended vertical/horizontal impact velocities and allowable G loadings. They are all higher than the speeds/Gs in the FAA crash design standards.
     
    Last edited: Nov 1, 2019
    Aerowerx likes this.
  6. Nov 1, 2019 #6

    bmcj

    bmcj

    bmcj

    Well-Known Member HBA Supporter

    Joined:
    Apr 10, 2007
    Messages:
    12,916
    Likes Received:
    4,862
    Location:
    Fresno, California
    For my own purposes, I would consider a crash to be an event whereby contact with the ground or other object that causes structural damage during the act of aviating (from the start of the takeoff roll to the end of the landing roll).

    Of course, there are outliers in (or out of) this definition, such as a hard landing that does not damage the airframe but causes injury to the occupants, or at the other extreme, collision with a bird that damages the wing or windscreen (and maybe even injures an occupant) but ends with a successful landing. Would either of these outliers be called a crash?

    And let’s not forget about the stock market “crash”. :p
     
  7. Nov 1, 2019 #7

    Aerowerx

    Aerowerx

    Aerowerx

    Well-Known Member

    Joined:
    Dec 1, 2011
    Messages:
    4,986
    Likes Received:
    1,357
    Location:
    Marion, Ohio
    Ok, so there is an accepted definition of "a crash" for small aircraft.

    As far as the FAA is concerned, any small aircraft should be built (I said should, not must) so the occupants will survive (but with what injuries??) under those conditions. Anything else and, well, good luck!
     
  8. Nov 1, 2019 #8

    Vigilant1

    Vigilant1

    Vigilant1

    Well-Known Member Lifetime Supporter

    Joined:
    Jan 24, 2011
    Messages:
    4,053
    Likes Received:
    1,796
    Location:
    US
    No, I didn't see that the document defines what a "crash" is. It's not defined in Appendix B ("Definitions"). And each G loading is accompanied by an expected injury level (given some assumptions)--but you'd have to dig for those. I don't think the criteria is "survival," it is more likely something like "significant injury expected above this load."
     
    Last edited: Nov 1, 2019
  9. Nov 1, 2019 #9

    Topaz

    Topaz

    Topaz

    Super Moderator Staff Member

    Joined:
    Jul 30, 2005
    Messages:
    13,732
    Likes Received:
    5,411
    Location:
    Orange County, California
    There is no perfect answer, nor a comprehensive set of criteria or practices that would cover every possible scenario. And there is always a tradeoff between performance and "safety."

    As a designer, you do your best to protect the largest number of people in the most-likely scenarios, and balance that goal against the performance that's required of the vehicle in everyday use. You're going to be unable to protect against some scenarios at all, unable to protect against others without crippling the utility of the airplane in its intended use, and there will be other cases where the world just develops a better idiot and you didn't see the case coming.

    If you design an airplane for serial production, even in small quantities, you have to accept the likelihood that someone is going to die or be injured in your design.

    And the first person who interprets this as me saying, "... so don't even bother designing for safety" gets banned from HBA for a week. (No, not really. But that would be an utterly incorrect interpretation of what I'm saying.)
     
    Pops and jedi like this.
  10. Nov 1, 2019 #10

    Pops

    Pops

    Pops

    Well-Known Member

    Joined:
    Jan 1, 2013
    Messages:
    7,131
    Likes Received:
    6,022
    Location:
    USA.
    Bob Barrows and I had a long discussion on this subject when I was designing the JMR. Some people can destroy a steel anvil.
     
    Last edited: Nov 2, 2019
    Topaz likes this.

Share This Page

arrow_white