This is a continuation of the previous blog with the same name, but labeled Part I.
The setting is the Hurricane Mesa Test Facility (HMTF) located atop Hurricane Mesa, a short drive from St. George, UT. HMTF has a 12,000 ft. test track that is fully capable of handling propulsion velocities exceeding supersonic. It is used mostly to test aircraft ejection seat systems, escape capsules, canopies, hatches, and other subsystems that require operation/actuation at specific speeds, during change of velocity at specific G forces, or under certain pitch and yaw conditions.
Figure 1 shows a cockpit test sled and pusher sleds. With up to 115 solid propellant rockets each generating 5000 lbs of thrust, a combined 575,000 lbs of thrust can be generated to achieve the desired speed for the test through the various pusher stages.
Figure 2 shows a zero-zero ejection test. A zero-zero ejection seat is designed to safely extract upward and land its occupant from a grounded stationary position (i.e., zero altitude and zero airspeed), specifically from aircraft cockpits. Zero-zero capability was developed to help aircrews escape upward from unrecoverable emergencies during low-altitude and/or low-speed flight, as well as ground mishaps. Before this capability, ejections could only be performed above minimum altitudes and airspeeds.
Zero-zero technology uses small rockets to propel the seat upward to an adequate altitude and a small explosive charge to open the parachute canopy quickly for a successful parachute descent, so that reliance on airspeed and altitude is no longer required for proper deployment of the parachute.
Substantial explosive activity is associated with an ejection, as can be imagined. The pilot has to be ejected to a height and direction appropriate for the parachute to open. The seat ejection, with the pilot attached, is designed to compensate for any pitch, roll, or yaw associated with the aircraft that could affect the ejection process. Additionally, for aircraft having a forward and rear seat, ejection has to be designed in such a way that the two do not collide during ejection. So, one is sent in one direction and the other in the opposite direction. Another interesting feature is that in a two-seat cockpit, the rear seat ejects prior to the forward seat by about 400 msec. If the forward seat would eject first, the rear seat would fry from the explosion and fire – an avoidable hazard that would not set well with the rear seat passenger.
So, who volunteers for these ejection tests? Or, to put it another way, who would be dumb enough to volunteer for this kind of ejection testing, and what about potential physical problems as a result of this rather violent activity, including acoustic trauma? Figure 3 shows the typical volunteer – a mannequin (it looks like a full-body distant cousin of KEMAR™ to those of us in hearing). Fortunately, none of the testing involves human subjects.
All of the ingredients are in place for potential hearing loss of an acoustic trauma origin. Even the pilot’s custom-fitted helmet that affords approximately 20 to 30 dB of passive NRR (noise reduction rating) attenuation would seem to be of marginal protection, especially since the lower NRR number relates to low frequencies, where most of the loud noise is likely to be located.
So, what does all this have to do with noise and hearing? Absolutely nothing.
In spite of the magnitude of the noise levels that are experienced, protecting hearing is not the primary objective during ejection. Saving lives is. Is it more important to save one’s hearing or save one’s life? I think that most pilots would take the latter. Comments from HMTF indicate that pilots today walk away from real-life ejections, although compression fractures of vertebrae are a potential side effect of ejection.
What Does All This Mean as it Relates to Hearing?
Sometimes we get so caught up in what we do (our disciplines) that we have difficulty imagining things outside the boxes we function in professionally. In other words, the world does not revolve around hearing – even though many hearing professionals will have a hard time accepting this.
Ejection seat testing is such a case where extreme noise levels occur and the ingredients are present for acoustic trauma. In spite of this, concern about hearing loss is essentially not an issue. When exposed to such trauma in real life, the purpose of an ejection seat is pilot survival. And being exposed to a typical acceleration of about 12-14g (117-137 m/s) during ejection creates its own problems unrelated to hearing.
However, to justify having read blogs Part I and Part II, this post did allow the reader to recall physics and acoustics principles and terminology. So, let’s just call it a refresher blog. And, sometimes, we have to surrender our professional involvements and accept conditions as they exist without hyping the fault – at least for now.