nema
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Dont Aircraft Controls Have An Error Overide?

Wed Jul 04, 2007 5:18 am

Ive just seen this picture on a.net that says the nose gear was accidentally retracted during boarding...

http://www.airliners.net/open.file?id=1230411

I am surprised that the controls would function to operate the gear up/down whilst on the ground.. is this correct?

Its a bit like being able to accidentally selecting reverse thrust when flying.
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ADent
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RE: Dont Aircraft Controls Have An Error Overide?

Wed Jul 04, 2007 7:31 am

Most airliners have landing gear squat switches to tell when weight is on the main gear.

I would assume the gear circuit would prevent a nose gear retraction in normal use when weight is on the mains.

But I would also assume there is an override for use when servicing the aircraft.

So either the airplane is not idiot resistant, the override was engaged, or the squat switch was broken, or something else. Also the caption may not be fully accurate and more scenarios come into play.

Quoting NEMA (Thread starter):

Its a bit like being able to accidentally selecting reverse thrust when flying.

That has happened too.
 
Yflyer
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RE: Dont Aircraft Controls Have An Error Overide?

Wed Jul 04, 2007 7:37 am

Hehe... Is anyone else reminded of that Simpsons episode where Homer gets mistaken for a pilot?
 
KELPkid
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RE: Dont Aircraft Controls Have An Error Overide?

Wed Jul 04, 2007 9:49 am

Quoting ADent (Reply 1):
or the squat switch was broken,

And Squat Switches (at least in GA planes) are notoriously unreliable  Wink I know of a Piper Arrow that pancaked while taxiing because the gear handle was up, the plane went over a bump in the taxiway, which unloaded the squat switch, and you know the rest of the story  Smile
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roseflyer
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RE: Dont Aircraft Controls Have An Error Overide?

Wed Jul 04, 2007 2:51 pm

Pretty much everything has an override that is used for testing. You can essentially trick the plane into thinking its in the air with the gear up while it is on the ground. This is used heavily in the manufacturing process. But it is also used in the day to day operations and maintenance. This is not an inherently dangerous situation, but when you get a breakdown in communication or inexperienced people working on planes, then you can have problems like a landing gear retraction on the ground.

But there is a protection. If you lift the landing gear lever while on the ground, the gear will not retract under normal conditions.
If you have never designed an airplane part before, let the real designers do the work!
 
ilikeyyc
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RE: Dont Aircraft Controls Have An Error Overide?

Wed Jul 04, 2007 3:19 pm

Quoting ADent (Reply 1):
Most airliners have landing gear squat switches to tell when weight is on the main gear.

I would assume the gear circuit would prevent a nose gear retraction in normal use when weight is on the mains.

But I would also assume there is an override for use when servicing the aircraft.

First of all, I believe the B767 uses proximity sensors, not switches, to detect a Weight-On-Wheels condition.

The aircraft I work on, when on the ground, can be tricked into thinking it is Weight-Off-Wheels just by pulling a few circuit breakers. Conversely, if it is jacked-up, it can be tricked into thinking it is Weight-On-Wheels by installing anti-targets near the proximity sensors.
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roseflyer
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RE: Dont Aircraft Controls Have An Error Overide?

Wed Jul 04, 2007 3:45 pm

Quoting Ilikeyyc (Reply 5):
The aircraft I work on, when on the ground, can be tricked into thinking it is Weight-Off-Wheels just by pulling a few circuit breakers. Conversely, if it is jacked-up, it can be tricked into thinking it is Weight-On-Wheels by installing anti-targets near the proximity sensors.

All modern Boeing aircraft work that way. The proximity sensors usually only detect the position of the landing gear. There are additional sensors if I remember correctly located just above the tires that detects if there is a load on the wheel.
If you have never designed an airplane part before, let the real designers do the work!
 
Tristarsteve
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RE: Dont Aircraft Controls Have An Error Overide?

Wed Jul 04, 2007 4:14 pm

Quoting RoseFlyer (Reply 4):
But there is a protection. If you lift the landing gear lever while on the ground, the gear will not retract under normal conditions.

The landing gear selector lever is protected on the ground by a spring loaded baulk that stops it being lifted. If you try and raise the lever nothing will happen. This baulk is removed when the aircraft is airbourne.
But there is an override lever or button right beside it. If you operate this button, then you can move the selector to up.
Moving the selector to up will have no effect unless the hydraulic system is pressurised. If it is the NLG will retract, and the MLG will try to retract.
The override lever or button is fitted for the rare case that, on take off the air/ground sense system fails to detect that the aircraft is airbourne, and there is an engine failure. With an engine failure on take off it is imperitive that the gear is retracted at once, as all performance calculations are based on this. Aircraft have a variety of features to ensure this can happen like PTUs in the hyd system.
If there is no engine failure, and the gear cannot be selected up, the crew should do some troubleshooting first because there may be a very valid reason for the baulk remaining in! On a 4 or 6 wheel bogie, the truck must have positioned to the flight position before the baulk is removed. This is to stop expensive graunching noises when the gear is retracted and misses the undercarriage bay!!!
 
fr8mech
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RE: Dont Aircraft Controls Have An Error Overide?

Wed Jul 04, 2007 4:54 pm

Quoting RoseFlyer (Reply 6):
There are additional sensors if I remember correctly located just above the tires that detects if there is a load on the wheel.

Nope, there are 2 sensors on each truck that sense tilt. There are 2 sensors at the nose that sense 'stut extended'. There are no sensors in the a/g system that sense weight or load on the wheels. This statement is valid for the B757/B767 aircraft.

In order to pull the gate, I believe, system 2 from each pair must be 'made'.
When seconds count...the police are minutes away.
 
Tristarsteve
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RE: Dont Aircraft Controls Have An Error Overide?

Wed Jul 04, 2007 5:11 pm

Quoting Fr8mech (Reply 8):
There are no sensors in the a/g system that sense weight or load on the wheels. This statement is valid for the B757/B767 aircraft.

But there are sensors on each MLG that sense that the strut is fully extended.
 
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HAWK21M
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RE: Dont Aircraft Controls Have An Error Overide?

Wed Jul 04, 2007 6:15 pm

Quoting Fr8mech (Reply 8):
Nope, there are 2 sensors on each truck that sense tilt. There are 2 sensors at the nose that sense 'stut extended'. There are no sensors in the a/g system that sense weight or load on the wheels. This statement is valid for the B757/B767 aircraft

The MLG has the sensors on the Truck between the MLG Wheels when tilted senses In Air condition.
regds
MEL
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fr8mech
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RE: Dont Aircraft Controls Have An Error Overide?

Thu Jul 05, 2007 3:00 am

Quoting TristarSteve (Reply 9):
But there are sensors on each MLG that sense that the strut is fully extended

I'll check the AMM, but I believ you are wrong.

Quoting HAWK21M (Reply 10):
he MLG has the sensors on the Truck between the MLG Wheels when tilted senses In Air condition

That's what I said. Each truck has 2 sensor that sense the tilt condtion of the truck.
When seconds count...the police are minutes away.
 
Tristarsteve
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RE: Dont Aircraft Controls Have An Error Overide?

Thu Jul 05, 2007 3:42 am

Quoting Fr8mech (Reply 11):
Quoting TristarSteve (Reply 9):
But there are sensors on each MLG that sense that the strut is fully extended

I'll check the AMM, but I believ you are wrong.

Quoting HAWK21M (Reply 10):
he MLG has the sensors on the Truck between the MLG Wheels when tilted senses In Air condition

That's what I said. Each truck has 2 sensor that sense the tilt condtion of the truck.

Ok on the B757/B767 there are two sensors that measure the truck tilt and give the air /ground sense.
On the A320 there are two sensors that measure that the torque links are fully extended and give air/ground sense.
On the B734 there are two sensors activated by a teleflex cable from the torque links that give air/ground sense.
On all aircraft there are two sensors that sense air/ground, it is just done in different ways.
Sorry you didn't like my simplification.
 
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HAWK21M
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RE: Dont Aircraft Controls Have An Error Overide?

Thu Jul 05, 2007 4:25 am

Quoting Fr8mech (Reply 11):
That's what I said. Each truck has 2 sensor that sense the tilt condtion of the truck.

9.6deg of tilt to be precise on the B752.
regds
MEL
I may not win often, but I damn well never lose!!! ;)

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