Citation :
Here is the essence of P4T's argument on the Flight Deck Door data & why it is wrong.
Thanks to the many folks who chased down the background for this, especially apathoid, 911facts and Reheat.
Many pieces & parts of this have already been posted here & elsewhere.
This is a my attempt at a summary.
This is a mixture of facts and high probability based on my engineering opinion. I've tried to identify each.
My opinions will get proven or disproven once someone has dug up Boeing's maintenance records on this particular plane.
Like Gaul (for you Latin fans), this is divided into 3 parts:
A. P4T's assertions
B. Direct response to their points, including their errors.
C. Some background info.
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A. P4T's stand:
(This is my interpretation from their posts, but I think that it's a fair one.)
1. They have generic documents that say that 757s are to be wired with door sensors.
2. The FDR has data inputs that are labeled "FLT DECK DOOR" for the cockpit door sensors.
3. Cap'n Bob's assertion that, if the sensor recording circuit were not wired to the FDR, then that input channel would be labeled "Spare", not "FLT DECK DOOR".
4. Cap'n Bob's assertion that "if it is on the plane, then it automatically becomes a required piece of equipment".
5. (Their latest addition at P4T) The fact that all the readings are Logical 0s implies to them that the door sensor was installed.
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B. Direct response to their points, including their errors.
For completeness, prior to each sentence, I have distinguished between "Fact" and those assertions that are my engineering "Opinion".
Referring to P4T's numbered assertions above:
1. [Fact ->] Generic docs don't mean anything. The early 757-1 and 757-2 planes did not have the sensors installed. The only documentation that proves that N644AA has been upgraded is a work-order with "N644AA" serial number at the top. This has not been provided.
2. [Fact ->] This is true.
3. [Fact ->] This is false. There were approximately 1066 inputs that were labeled. Only 88 were required by the FAA. [Opinion ->] The other 978 were chosen by the airline engineering staff as "most useful to be able to monitor for ourselves or our customers". [Fact ->] At least 24 (& perhaps as many as 44) were set aside as "Spares".
4. This is a fundamental mistake that has been pointed out to Cap'n Bob. And yet he keeps repeating it. [Fact ->] The actual FAA policy that he is intentionally misquoting is "If it is on the plane, it must be FUNCTIONAL". If it is not functional, then [according to the FAA's 14 CFR 91.312(d)], there is a strict procedure that allows only "non-essential to flight safety" parts to be a) removed or b) disabled & "placarded as non-functional". Essential parts must be fixed before the plane can fly.
Bob is suggesting that, because there is an FDR input channel labeled "Flight Deck Door", then that system must be installed & hooked up to the FDR. [Fact ->] This is utterly wrong. The FDR channel allows the "capability for that cockpit monitoring circuit" to be recorded by the FDR. It does not qualify as proof that the system itself is installed on the plane.
In a similar fashion, wiring harnesses for the cockpits of commercial jets are fabricated with the capability of accommodating many customer options for gauges, auto-pilot systems, etc. The "full capacity" wiring harnesses are installed (including unused & labeled connectors) to reduce the number of custom harness configurations, to ease the process of adding these options on the manufacturing floor, and to ease the process of upgrading the plane later. The existence of these unused wire harness connectors does NOT imply the existence of these uninstalled components, any more than the existence of an allocated FDR channel for "Flight Deck Door" implies the existence of a cockpit door monitoring circuit.
[Fact ->] Note that, if Robby's assertion were true, then every single plane that used the Fairchild FDR would be required to wire approximately 1000 sensors into the plane. Because that is approximately how many labeled FDR input channels are in that system.
[Opinion ->] The reason that these FDR inputs have those labels is that Boeing standardized this FDR for their fleet, and retrofitted older planes with it. [Fact ->] The new FDRs have the capacity to monitor this info (and lots more), but the older planes are not wired to provide the info unless they have been individually upgraded.
Bob also [Opinion ->] intentionally misleads nontechnical readers by confabulating a circuit that can announces an open cabin door using a warning light in the cockpit with a circuit that sends this info to the FDR. [Fact ->] These two circuits are entirely independent of each other.
5. [Fact ->] This is a giant, unsubstantiated piece of guess-work on the part of an auto mechanic who stands in as P4T's aviation expert. [Opinion ->] And is fundamentally wrong. [Fact ->] The answer to this question is contained in Boeing's wiring & installation diagrams for this FDR.
[Fact ->] Good engineering practice requires that electronic monitoring circuits not allow their inputs to "float", i.e., remain unconnected. If that happens, then the state of that circuit can bounce up & down, producing "hash" (i.e., noise & crosstalk) within the circuits & possibly disturb other inputs. Standard practice is to tie all unused analog inputs to a common return, and all unused digital inputs to logical 0, in both cases using "pull down resistors". If the particular circuit is later installed, then the pull-down resistor is removed & discarded when the sensor inputs are hooked up.
The above addresses ALL of P4T's claims about this "mysterious, incriminating data". There is nothing mysterious or incriminating about it. It is exactly the situation that one would expect when maintaining a fleet of airplanes that had a 20+ year manufacturing life and multiple upgrade production cycles.
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C. Some background.
Prior to 1997, the FAA required only a few key flight parameters to be recorded. In 1997, they increased the number of required recoded parameter to 88. [See below for a link & list of parameters.] There was no requirement for monitoring the cockpit door.
Note that, even in this group, thirty (30) of those parameters were only required "if the information source was installed". For example, if there was no Radar Altimeter, then one did not have to be installed.
N644AA was upgraded to the Post 97 wiring requirements, per its FDR data (see "Post 97" data bit indicated, on FMC & DocData Parameters chart, pg II-15 of NTSB Specialist's Factual Report of Investigation of DFDR, DCA01MA064 (1/31/02)
This upgrade still did not require cockpit door monitoring.
3. The airlines were given the 88 parameters that were required by the FAA. They were perfectly free to record any additional ones that they, or their customers, chose.
According to the NTSB's DCA01MA064 document, the AA77's flight recorder had approx 336 parameters "confirmed working", and approx 730 were either "not working or unconfirmed" at the time of this report (1/31/02). It is known that some of the data in the "not working or unconfirmed" category was later decoded (e.g., the radar altitude).
In addition to the channels above, there were 14 spare analog channels (10 bit) and 10 spare digital channels (1 bit), listed on the 737-3b_1.txt document, but not included in the DCA01M064 Document.
Boeing's 757-3b data frame document says that there are 1110 parameters in the report. With approximate counts, I've accounted for 1090 of them. Someone who has time & interest in refining this number can find the remaining 20 if they wish. I suspect that they may also be "spares".
Required parameters:
Per 14 CFR 121.344
http://www.flightsimaviation.com/dat...t_121-344.html
Note that N644AA had most or all of these instruments installed. Note that more than 88 channels would be required (approximately 100). While most of these items require one input channel each, some parameters (1, 85 - 88) require more than one channel each.
(1) Time;
(2) Pressure altitude;
(3) Indicated airspeed;
(4) Heading -- primary flight crew reference (if selectable, record discrete, true or magnetic);
(5) Normal acceleration (Vertical);
(6) Pitch attitude;
(7) Roll attitude;
(8) Manual radio transmitter keying, or CVR/DFDR synchronization reference;
(9) Thrust/power of each engine -- primary flight crew reference;
(10) Autopilot engagement status;
(11) Longitudinal acceleration;
(12) Pitch control input;
(13) Lateral control input;
(14) Rudder pedal input;
(15) Primary pitch control surface position;
(16) Primary lateral control surface position;
(17) Primary yaw control surface position;
(18) Lateral acceleration;
(19) Pitch trim surface position or parameters of paragraph (a)(82) of this section if currently recorded;
(20) Trailing edge flap or cockpit flap control selection (except when parameters of paragraph (a)(85) of this section apply);
(21) Leading edge flap or cockpit flap control selection (except when parameters of paragraph (a)(86) of this section apply);
(22) Each Thrust reverser position (or equivalent for propeller airplane);
(23) Ground spoiler position or speed brake selection (except when parameters of paragraph (a)(87) of this section apply);
(24) Outside or total air temperature;
(25) Automatic Flight Control System (AFCS) modes and engagement status, including autothrottle;
(26) Radio altitude (when an information source is installed);
(27) Localizer deviation, MLS Azimuth;
(28) Glideslope deviation, MLS Elevation;
(29) Marker beacon passage;
(30) Master warning;
(31) Air/ground sensor (primary airplane system reference nose or main gear);
(32) Angle of attack (when information source is installed);
(33) Hydraulic pressure low (each system);
(34) Ground speed (when an information source is installed);
(35) Ground proximity warning system;
(36) Landing gear position or landing gear cockpit control selection;
(37) Drift angle (when an information source is installed);
(38) Wind speed and direction (when an information source is installed);
(39) Latitude and longitude (when an information source is installed);
(40) Stick shaker/pusher (when an information source is installed);
(41) Windshear (when an information source is installed);
(42) Throttle/power lever position;
(43) Additional engine parameters (as designated in Appendix M of this part);
(44) Traffic alert and collision avoidance system;
(45) DME 1 and 2 distances;
(46) Nav 1 and 2 selected frequency;
(47) Selected barometric setting (when an information source is installed);
(48) Selected altitude (when an information source is installed);
(49) Selected speed (when an information source is installed);
(50) Selected mach (when an information source is installed);
(51) Selected vertical speed (when an information source is installed);
(52) Selected heading (when an information source is installed);
(53) Selected flight path (when an information source is installed);
(54) Selected decision height (when an information source is installed);
(55) EFIS display format;
(56) Multi-function/engine/alerts display format;
(57) Thrust command (when an information source is installed);
(58) Thrust target (when an information source is installed);
(59) Fuel quantity in CG trim tank (when an information source is installed);
(60) Primary Navigation System Reference;
(61) Icing (when an information source is installed);
(62) Engine warning each engine vibration (when an information source is installed);
(63) Engine warning each engine over temp. (when an information source is installed);
(64) Engine warning each engine oil pressure low (when an information source is installed);
(65) Engine warning each engine over speed (when an information source is installed);
(66) Yaw trim surface position;
(67) Roll trim surface position;
(68) Brake pressure (selected system);
(69) Brake pedal application (left and right);
(70) Yaw or sideslip angle (when an information source is installed);
(71) Engine bleed valve position (when an information source is installed);
(72) De-icing or anti-icing system selection (when an information source is installed);
(73) Computed center of gravity (when an information source is installed);
(74) AC electrical bus status;
(75) DC electrical bus status;
(76) APU bleed valve position (when an information source is installed);
(77) Hydraulic pressure (each system);
(78) Loss of cabin pressure;
(79) Computer failure;
(80) Heads-up display (when an information source is installed);
(81) Para-visual display (when an information source is installed);
(82) Cockpit trim control input position -- pitch;
(83) Cockpit trim control input position -- roll;
(84) Cockpit trim control input position -- yaw;
(85) Trailing edge flap and cockpit flap control position;
(86) Leading edge flap and cockpit flap control position;
(87) Ground spoiler position and speed brake selection;
(88) All cockpit flight control input forces (control wheel, control column, rudder pedal).
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