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Old 30th Aug 2008, 15:32
  #1306 (permalink)  
Join Date: Mar 2003
Location: BC
Age: 72
Posts: 2,410
I would think, the MD82 not being FADEC equipped, there would be a provision in the mechanical interlock to retard the thrust lever to idle in case of an uncommanded TR deployment. Apparently this is not the case on the MD82.

Also I understand there is a difference in operational philosophy between the type I have worked on (Fokker 70 and 100) which inhibit certain alerts- and the MD8x series which apparently do not inhibit certain alerts during some stages of the takeoff phase.
Re worries - it's one big dialogue, out there on the table!

To my knowledge, there's no such mechanical interlock which would pull a throttle to IDLE should a reverser accidently deploy. The other side of such a "protection" is the accidental operation of such a system at an inappropriate time...the same logic (and thinking) which is being used in places to discuss an "accidental" deployment applies to all systems including one which is designed to retard a throttle to IDLE - in considering the design of such a system, would it be an actual mechanical link between reverser (which bucket? to the throttles, (very difficult and not reliable), or sensor-driven (pressurization of the hydraulic cylinders which deploy either reverser)? While the problems aren't insurmountable, is the need there? Would the actuators within the pedestal or below the floor/pedestal be clutches (part of the MD80 autothrust system) or separate bell-cranks/cables? On the Airbus, FADEC simply commands IDLE, (not comparing types here, please) so should such a system be software-driven?

I'm not an engineer but I know the outcomes (error-tolerance) of both brittle (those which must work perfectly if they are to work at all) and resilient systems (those which tolerate hardware/software error but still respond successfully). Designing a reverser system that is both resilient and does it's job well without undue risk to the aircraft is a challenge for the engineers. The prevention of accidental deployment in such designs rather than designing a system with "the aftermath" of such deployment in mind, has been a decision of the industry. Accidental deployment is generally viewed, with justification, as a rare though not impossible event- the choice of the industry would be based upon experience, capabilities, weight, complexity, reliability and costs. The reverser systems on the MD80 fleet type are essentially the same as the B737 fleet type - they've been around a long, long time and they're successful systems as proven by the stats...(someone posted a list of such deployments a few pages back - remarkable record really, when one considers perhaps a billion flights in these types from the mid-60's to today with the same basic design). Likely the decision regarding throttles-to-idle was made in this context - rarity, need and cost.

Regarding inhibition of warnings, I can't speak to the MD80 type; the DC9 may have but I can't recall - it was a superb, but pretty basic airplane at the time as was the 737 design. The Airbus 320 type does inhibit non-emergency/non-essential messages according to flight regime. A reverser-unlocked message is inhibited until 1500' on the Airbus, likely because they consider the airplane controllable - also, it was certified this way, with the same assumptions. Again, in the sim it is something one would need to be aggressive with but yaw and roll are not uncontrollable. Again too, FADEC reduces engine thrust to IDLE immediately.

These issues are all part of the certification process for a particular design. Once such criteria are established, it all comes down to training for such events.

On that point, I would like to digress a moment.

Perhaps out of this thread, among many benefits for those who persist and read but who aren't in aviation, is a notion of how often the cost-benefit equation comes up.

Perhaps out of such (rare, thankfully) public dialogues, people who want a ticket for 20 bucks may come to realize that like their car, their house and perhaps their life, airplanes and their operation, through millions of decisions, are one huge compromise and not the perfect machines (nor the perfect systems) that people assume.

They might also begin to realize that their comfortable, warm seat in a machine that is doing almost a thousand feet per second doesn't come without a price and that "29 Euros, dollars, etc" simply can't sustain a viable, safe aviation enterprise for long.

The potential for some appreciation of this point is present in this and other threads - those who literally whine about the price of an airline ticket by bringing their sense of entitlement to low prices from the department store to an airline need to consider their complaints and perhaps even hypocritical commentary within this very important context.

The angry, innocent and, after an accident, entirely reasonable question, "why?", will always have several components where failure first manifests, and which, until the moment of the accident, may remain known but unaddressed by cost-conscious managers who are trying to do their "very best" for shareholders.

Flight safety departments and programs are increasingly expected, by new-age, MBA'd managers, to be "profit centers". Flight Safety departments, if they exist at all at an airline, are expensive and, to the bean-counters and flight operations managers who spend too much time flying a desk, produce "nothing" and are therefore difficult to justify in both good and bad times.

Flight Safety departments are not "profit centers" however, unless the protection of the shareholder investment and keeping people safe in airplanes is accepted as contributing to profit by not causing lossess.

Such departments are iintended to be an independant intervenor when the commercial aspect of an enterprise takes too high a priority over risk but too often, such departments are seen as backwaters - as career-ending dead-end paths in a corporate world, instead of sharing center-stage with those departments which must make sure that costs are controlled.

Airlines teach the public that they can do this cheaply; "lo-cost" can be done, and safely, but the principles of aviation do not go away; they cost money and it takes more than twenty Euros/dollars to do the work.

The "inventory" for an airline is time and an empty seat which are both ephemeral ! - THAT is all an airline has to sell, (service, of course, but who's travelled lately?!). Once the airplane door is shut, those two "commoditites" disappear forever, irrecoverable assets lost to time but which have had to be paid for nonetheless. Thus the squeeze. Thus the need to fill every seat and even then, a full flight may not be breaking even.

These are the economic realities of all airlines. The next time one complains about the price of a ticket, consider that you are not buying a ticket from A to B. You are paying thousands of people dedicated to your safe arrival, a few pennies each for their services.

Sorry for the diversion...

Isn't it possible that the flaps/slats were extended in a recovery effort, but too late? That would explain finding the slats extended.
I think the times involved (seconds) and the circumstances faced by the crew (which, if the slats/flaps were retracted, would not be apparent until rotation/liftoff) would not permit such action, nor would the surfaces achieve their extended position as shown, in the time available.

Last edited by PJ2; 30th Aug 2008 at 16:06.
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