Cdfmxtech From United States of America, joined Jul 2000, 1338 posts, RR: 28 Posted (12 years 6 months 3 weeks 5 days 15 hours ago) and read 6384 times:
B737-300/400/500 Engine Fire Detection
It says that an ovht condition is set when the first alarm setting is reached. A fire condition is set when the second alarm setting is met. I understand the whol resistance/temperature relation ship. Schematics show a dashed line leading to ground above and below a solid line...which is the core. Does that mean that when the resistance level is the highest, the ground is met...or is the ground met at the first alarm setting. I always thought that the box in the E&E is monitoring resistance levels - acting as an ohmmeter of sorts. When the resistance level meets the first number, OVHT happens. When the resistance level reaches the second number, the FIRE condition happens. Somebody help
Airplay From , joined Dec 1969, posts, RR: Reply 3, posted (12 years 6 months 3 weeks 5 days 11 hours ago) and read 6284 times:
The heat/fire detector measures capacitance of the loop. When the material separating the center and outer conductors is heated, it's dielectric properties are changed, effectively changing the capacitance of the loop.
Some of the older graviner units (Kidde bought Graviner) used to be really sensitive to water and short circuits. The systems have been since improved not to respond as easily to resistive interference like water.
HAWK21M From India, joined Jan 2001, 31457 posts, RR: 57 Reply 4, posted (12 years 6 months 3 weeks 5 days 3 hours ago) and read 6276 times:
Your right, its KIDDIE fire detectors,
There is an INCONEL tube filled with a chemical whose property is such that as Temperature increases the resistance of that chemical decreases [Resistance inversely proportional to Temperature].
One ground wire is connected to the outside of the Tube,while the 2nd wire is in the centre surrounded by this chemical.
Under normal circumstances the resistance is high enough to prevent current flow between the two wires, as the temp rises the chemical resistance decreases which breaks the barrier between the two wires & this continuity is sensed by a Fire detection unit to illuminate the appropriate warning.
The difference being that there are two inconel wire sets for the fire detection system with the circuit set to permit current transfer at a particular temp.
On the -200s its 400degF for OVHT & 600degF for FIRE warning.
JT-8D From United States of America, joined Dec 2000, 423 posts, RR: 3 Reply 6, posted (12 years 6 months 3 weeks 4 days 12 hours ago) and read 6266 times:
OK, heres my 2 cents. On a typical fire/oh detection system there are two identical loops. If one shows an overheat condition, it will show o/h in the cockpit. If both show o/h, a fire will be indicated. The only systems Im aware of that show "varying " resistance is the lindberg system (made now by systron donner). In this system a gas filled tube is attached to a set of pressure switches. Low pressure=fault indication, Higher pressure=overheat, really high pressure=fire. The fire detection accy box monitors the whole system and can interpret what is a fault and what is real. The Kiddie/Fenwall loop systems read either open or closed. The older planes used the "salt" type loops, while the newer (737 ng) uses the gas type(lindberg). By salt I mean eutectic salt(excuse spelling) that drops its resistance at a certain temp..JT
Chdmcmanus From United States of America, joined Mar 2001, 374 posts, RR: 2 Reply 7, posted (12 years 6 months 3 weeks 4 days 11 hours ago) and read 6258 times:
I agree with JT. The old -141 uses Systron-Donner (Helium pressure) on the engines/pylons, and Kiddie inconnel (resistance) on the wings/apu. To add to what he said about the SD, it has a pinpoint and overall heat setting.
Pylon avg 400F, discreet 950F
Fwd Cowl avg 350F, discreet 1050F
Aft Cowl avg 500F, discreet 1050F
The inconells are 195/205 C for the anti-ice, 310 F wing bleed air, and 232 C Apu, both of which only have one indication, either "on" or off".
This isn't the 747, but the principle is the same.
Cdfmxtech From United States of America, joined Jul 2000, 1338 posts, RR: 28 Reply 8, posted (12 years 6 months 3 weeks 4 days 7 hours ago) and read 6253 times:
I disagree. A system such as the engine fire detection system on most aircraft are labled A/B as you know. If tyhe sel is in the both position, then it will take a signal from both loops to get an indication...whether it be ovht or fire. Isolate it to A or B and an indication in either one will set the alarm.
Maybe the manual can say it a little better than I can...
The overheat detector (OVHT DET) switch on the overheat/fire protection panel lets the crew select the mode of operation. Each switch has these positions:
· NORMAL, loop select logic makes loop A and loop B agree before the alarm condition operates (AND logic).
· A, loop select logic makes loop A the only sense authority (OR logic).
· B, loop select logic makes loop B the only sense authority (OR logic).
JT-8D From United States of America, joined Dec 2000, 423 posts, RR: 3 Reply 9, posted (12 years 6 months 3 weeks 4 days 4 hours ago) and read 6252 times:
I didnt address operation of the control panel, but I agree with your last post. If you only have 1 good loop, then one loop can cause a fire indication if the switch is set to A or B. The NG airplanes will automatically switch to single loop mode if it sences a fault. The older planes had to be switched manually..JT