@lomapaseo
How does 100-200gs on a persons body equate to G loadings on a CVR/FDR? innards
How does 200 atmos pressure at the bottom of the sea cause damage to a memory chip in a module if it is exposed to water intrusion?
If I deal with your question on pressure first; at 4000m, the pressure at that depth is 398atm, or 403 bar, or 411 kgf/cm2, or 5846psi, or 2.9tons/sqin. Starting from the point that a hollow object if present in a fluid at high pressure, is filled with air at normal atmosphere, and has a small hole, lets say a microfracture, water will enter, albeit veryslowly, but enter it will, eventually compressing the air to 1/400 it's volume and replacing the balance with water. Integrated circuits of the old plastic dual-in-line through hole type are formed by a solid layer of resin plastic which covers the entire microcircuit die. Ceramic devices (wide temp spec types -40 to +125C for the military) have a cavity above the microcircuit which if subject to high pressure is prone to fracture. Similarly, the small Surface mount devices (like we see in mobile phones), have an enclosed void under the die inside the resin, which again is subject to failure under pressure. Now, if the FDR/CVR armoured case leaks, water will inexorably progress into all parts of the interior, around the electronics, and cover The memory devices until, when the flow has stopped and there is no differential to the outside, they too are subject to the same pressure that exists outside of the cylinder. If the memory device uses a package which has a void, then it too may suffer a stress fracture, and again, water will find it's way in until the pressure differential goes to zero. Plastic IC's are now much more prevalent in aerospace than I at first thought. That being so, if plastic parts are used, then the porosity of the plastic will allow ingress of water without the need for any fracture event. The links below show how fractures may occur when IC's are stored in a damp environment and then suffer damage during the heat treatment process required to mount them on PCBs. In this case at far lower pressures since the material is in tension due to expansion, whereas, in compression they are much stronger.
http://nepp.nasa.gov/docuploads/A178...ems_final2.pdf
Moisture absorption
Regarding the question of equating acceleration on a person's body and that experienced by the FDR, and by implication, the CVR. The summarised findings of BEA's pathology report suggests a more or less horizontal impact. White's study (1993 The effects of structural failure on injuries sustained in the M1 Boeing 737/400 disaster) used injury analyses to allow direct comparisons with other linear measurements of a similar scale to show that severity of the injuries mirrored the damage to the aircraft. Similar suggestions were made by Cullen (NATO - Injury Mechanisms in Aircraft Accidents ) who concluded that the method be considered in the analysis of accidents where it is not possible to examine the crashed aircraft, such as accidents over ocean. The manual for civil aviation medicine quotes: Examination of the bodies of passengers can establish a pattern of injuries. Such a pattern may be uniform or discordant. A uniform pattern suggests that all the passengers were subjected to much the same type and degree of force.
For an accident over terrain, the forces acting upon the occupants are frequently less than those applied to the aircraft ie; the structures may show more damage than the injuries to the occupants suggest since the aircraft structures absorb energy as the collapse progresses. A water impact is different, and passengers are likely to be subjected to higher forces for the reasons explained before, namely; failure of the buckling absorption process in the structural members, and hydraulic shock to the floor structure. Once the skin fails, the drag profile changes as does the loading on the structural members. The floor then takes the brunt, and when that fails, the upper outer skin panels.
Passengers and FDR/CVR are, if I recall the details of a photo in an earlier post correctly, located at similar heights above the aircraft datum lower fuselage - maybe the FDR/CVR are a bit higher, though with a slight pitch up as mentioned in the report, this might have put the FDR/CVR to the same level or lower. So, as a first approximation, if we take horizontal impact on terrain as an exemplar, the stopping distance is similar, with the frames further forward of the empennage taking the energy until either the empennage contacts the ground or breaks away at a frame section. Hence my comment; the FDR and CVR may have experience something similar or less.
It can be argued that a small equipment object like the FDR could experience any number of pathways to final stop (air/water boundary), and that trying to predict how energy is absorbed and max G is fatuous. However, in this instance we are dealing with fluid dynamics where for a small object (low cross sectional area), the effects of drag, and by implication, retardation force is likely to be lower than that experienced by passengers subject to a hydraulic surge which pushes up against the large surface area of the floor. Examining the FDR photo I note that there is some damage at two points to the end of the casing consistent with contacting a hard edge, probably when the pinger departed. What is more interesting is the way the support fixings for the memory cylinder have failed. On the top two lugs where the earth strap is located, it looks like both bolt heads have popped off, while for the lower two, they look as if they are distorted - slightly bent. This suggests to me that the cylinder received a force low from one side which was sufficient to lift the unit, causing the bolt heads to fail, and bending away from the chassis (possibly explaining the chassis distortion) until the lower bolts failed, but not before bending sufficiently to remain trapped in the cylinder supports. Yet, if this was the case, then where is the associated impact mark on the cylinder paintwork? One hypothesis is that the cylinder was detached from the chassis as a result of fluid forces, and since the rising edge (dp/dt) of a fluid pressure pulse is less severe than a hard object impact, and the deceleration on a small object correspondingly less.
Could the cylinder have hit something else, the chassis for instance, and experienced a high G from that source - the answer is yes, of course. However, unlike the passenger G event, which might occur over a longer period, anything from say 100 to 300 ms, with a vertical vector, a detached memory cylinder is likely to experience only brief interactions, and varying vectors mitigated by fluid drag. So it's quite possible that the FDR experienced less G than the passengers.
Note: for ChrisN - Guidline from manual:
Injury sustained Deceleration
Nose - fracture 30G
Vertebral body - compression 20-30G
Fracture dislocation of C1 on C2 20-40G
Mandible - fracture 40G
Maxilla - fracture 50G
Aorta - intimal tear 50G
Aorta – transection 80-100G
Pelvis – fracture 100-200G
Vertebral body – transection 200-300G
Total body fragmentation >350G
Apologies for the length...