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QUESTION: 737 Landing Gear Doors  
User currently offlinenickh From United States of America, joined Jun 2008, 200 posts, RR: 0
Posted (3 years 7 months 5 days 4 hours ago) and read 6466 times:
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I have a question about something strange that I saw the other day:

I was driving down a major thoroughfare here in Houston, Texas, which just happens to be under the Primary Approach Path to HOU (Houston, William P. Hobby Airport) - mostly frequented by Southwest Airlines.

So - As I was driving along, and I had encountered several stop lights along the way, I noticed two SWA 737s on final to HOU (from where I was, the 737s were still about 25-miles out) and while the first one was flying a standard approach, the second that followed a few miles back, had something interesting to observe:

I noticed that on the second 737, the main landing gear was UP, however, the front landing gear doors were OPEN -- BUT the front landing gear was NOT lowered.

I was at a stop light for about a minute and a half, so I watched the progress of the aircraft (with the sunroof on my car opened) - and I could see that the aircraft was flying for some time with the front gear doors opened, but no landing gear in sight.

So - the question. How can this be possible? As a G/A pilot myself, no aircraft that I know of can allow you to open the gear doors (manually?) but NOT lower the gears. Unless there is something that I do not know.

Any comments/thoughts are appreciated.

Regards,

-Nick


"We all have wings, but some of us don't know why..."
16 replies: All unread, jump to last
 
User currently offlineKELPkid From United States of America, joined Nov 2005, 6388 posts, RR: 3
Reply 1, posted (3 years 7 months 5 days 2 hours ago) and read 6395 times:

...and as a GA pilot (with lots of complex & high performance time), I can assure you that when the microswitches act up in the wheel wells of Cessnas, that yes, indeed, the doors can sometimes remain opened in flight   I was on board a 210 once with my instructor, and we had to land with the main gear doors open because the gear system was acting up. Destroyed the doors when we drug them along the runway during the flair   Had no choice, our in-flight gear fiddling yielded no results.

Of course, Boeings have forsaken unreliable and failure prone microswitches for better technology years ago. However, I would be willing to wager that what you saw was probably a failure of something-definitely not a normal state of affairs.



Celebrating the birth of KELPkidJR on August 5, 2009 :-)
User currently offlineN901WA From United States of America, joined Oct 2009, 463 posts, RR: 0
Reply 2, posted (3 years 7 months 4 days 22 hours ago) and read 6324 times:

That is strange on a 737. The 737's I have worked on the rods that open and close the nose gear doors are connected to the upper nose gear and thru a crank to the doors. To have both open with the nose gear up, then they must have been wrong. Here's a picture I took tonight to show what I am talking about. Both doors run independent and has no summing rod. HTH


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 3, posted (3 years 7 months 4 days 21 hours ago) and read 6290 times:

Quoting KELPkid (Reply 1):
Of course, Boeings have forsaken unreliable and failure prone microswitches for better technology years ago.

Actually, most OEM's forsook unreliable and failure prone microswitches for unreliable and failure prone proximity switches decades ago...although better, it's far from a perfect system.

Tom.


User currently offlineLimaFoxTango From Antigua and Barbuda, joined Jun 2004, 789 posts, RR: 1
Reply 4, posted (3 years 7 months 4 days 20 hours ago) and read 6251 times:

Quoting nickh (Thread starter):
So - the question. How can this be possible? As a G/A pilot myself, no aircraft that I know of can allow you to open the gear doors (manually?) but NOT lower the gears. Unless there is something that I do not know.

The alternate extension system for the DHC8 landing gear technically allows you to open the doors without lowering the gears. The question would then be WHY would you want to do such? Putting that aside, it CAN be done.



You are said to be a good pilot when your take-off's equal your landings.
User currently offlineRoseFlyer From United States of America, joined Feb 2004, 9643 posts, RR: 52
Reply 5, posted (3 years 7 months 4 days 19 hours ago) and read 6212 times:

The 737 cannot lower doors without the gear unless something unusual happened. The doors are connected by slave linkages to move with the gear during normal and manual extensions. It's a clever design that eliminates the need for door actuators or door locks.

If the doors were open, I am guessing something was broken on the airplane. The doors are independent of each other and both directly connected to the gear at the shock strut. It surprises me if they were both open, but you never know what can happen.

A complete hypothesis on my part is that the gear doors were re-rigged recently. If someone did not put an adequate preload on the door operating rods and just rigged them to be shut and did not adequately engage the aerodynamic seals, then aerodynamic forces could have forced the doors open in flight. I assume the pilots would notice the noise of that happening, but there is no indication in the flight deck. Regardless the penalty is only drag. The plane is perfectly safe to fly.

Quoting KELPkid (Reply 1):
Of course, Boeings have forsaken unreliable and failure prone microswitches for better technology years ago. However, I would be willing to wager that what you saw was probably a failure of something-definitely not a normal state of affairs.

737 doors are purely mechanical driven. No switches whatsoever.



If you have never designed an airplane part before, let the real designers do the work!
User currently offlineAA737-823 From United States of America, joined Mar 2000, 5824 posts, RR: 11
Reply 6, posted (3 years 7 months 4 days 12 hours ago) and read 6087 times:

Greetings from a 737 mechanic, here.

Quoting LimaFoxTango (Reply 4):
The alternate extension system for the DHC8 landing gear technically allows you to open the doors without lowering the gears. The question would then be WHY would you want to do such? Putting that aside, it CAN be done.

Not on a 737, it can't.

Quoting RoseFlyer (Reply 5):
The 737 cannot lower doors without the gear unless something unusual happened. The doors are connected by slave linkages to move with the gear during normal and manual extensions. It's a clever design that eliminates the need for door actuators or door locks.

Indeed.
Which is why the OP was either observing an optical illusion, or...... something.
If a rod broke, the aerodynamic forces would hold the door CLOSED, not open.

But the 737 has the simplest landing gear design of any modern airliner... there really aren't any options or failure modes.


User currently offlinetdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 79
Reply 7, posted (3 years 7 months 4 days 9 hours ago) and read 6012 times:

Quoting RoseFlyer (Reply 5):
737 doors are purely mechanical driven. No switches whatsoever.

Plenty of prox switches on the indication side...not on the actuation side though, as you correctly note.

Tom.


User currently offlineSEPilot From United States of America, joined Dec 2006, 6910 posts, RR: 46
Reply 8, posted (3 years 7 months 4 days 9 hours ago) and read 6010 times:

Quoting tdscanuck (Reply 3):
Actually, most OEM's forsook unreliable and failure prone microswitches for unreliable and failure prone proximity switches decades ago...

When I was designing grinding machines in the late 70's and early 80's proximity switches were just becoming widely available. Our instructions from management were to only use them when mechanical switches wouldn't work. Within a short time that order was reversed; use proximity switches as the first choice. I do not recall any case where we had a problem due to proximity switches failing.



The problem with making things foolproof is that fools are so doggone ingenious...Dan Keebler
User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 9, posted (3 years 7 months 4 days ago) and read 5890 times:

On the B737s the NLG doors are mechanically operated through pushrodes/Bellcranks vide the Trunion link rotation & match the gear Extend/Retract sequence.
Your observations if accurate points to a mechanical problem with the NLG doors.

regds
MEL.



Think of the brighter side!
User currently offlinenickh From United States of America, joined Jun 2008, 200 posts, RR: 0
Reply 10, posted (3 years 7 months 3 days 5 hours ago) and read 5665 times:
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Thanks for the replies.

Still, what strikes me as odd - that SWA 737 being so close to final approach, unless they were going around and maybe I just happened to catch that second 737 in a holding pattern - perhaps they lowered, but then raised the gears (?) and maybe the hydraulics were slow and I just happened to see it with the gears up, but the doors open at just the right 1.5-minute time-frame, that is the best speculation that I can come up with.

I've flown out of HOU since 1989 and have been around a lot of 737s - the Nose Gear Doors on this particular 737 in question, were open for an abnormally long period of time with no Nose Gear.

Well, there were no local accident reports, so I am guessing that it was just an anomaly. Still, it was a strange thing to see.

Thanks for the comments.

Regards,

-Nick



"We all have wings, but some of us don't know why..."
User currently offlinebio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Reply 11, posted (3 years 7 months 2 days 16 hours ago) and read 5536 times:

Quoting nickh (Thread starter):
So - the question. How can this be possible?

If BOTH doors were open, It's possible with BOTH linkage rods broken or disconnected - these should be independent faults since each door has its own linkage rod mechanism.

You can't tell from the flight deck if those doors are open. The green or red "NOSE GEAR" indicator lights only refer to the nose landing gear position, but not the doors. Since it's a mechanical linkage only, you assume that a "NOSE GEAR" green light indicates the nose gear is up and locked AND the doors closed. I'ts important to look at these linkages during the exterior inspecion because if one of them is broken you will be flying with that door open.

----------

I can't find any info on the DDG or CDL that includes the nose landing gear doors, other than the nose gear door rubber seals on the CDL, so I'm not sure if the rods can be disconnected with the doors locked in the open position by any maintenance procedure. I haven't found any info on fuel penalties either.



Alfredo


User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 12, posted (3 years 6 months 4 weeks 1 day 1 hour ago) and read 5256 times:

Quoting bio15 (Reply 11):
you assume that a "NOSE GEAR" green light indicates the nose gear is up and locked AND the doors closed.

On B737....

Nose Gear Red Light indicate,gear unsafe position or up [depends on situation]
No lights indicates Gear up & locked.
Nose Gear Green light indicates Gear Down & Locked.

regds
MEL.



Think of the brighter side!
User currently offlinebio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Reply 13, posted (3 years 6 months 4 weeks 19 hours ago) and read 5244 times:

Quoting HAWK21M (Reply 12):
Nose Gear Red Light indicate,gear unsafe position or up [depends on situation]
No lights indicates Gear up & locked.
Nose Gear Green light indicates Gear Down & Locked.


Yes, my mistake typing! I was supposed to say:

"NOSE GEAR" lights OFF indicate the gear is up and locked. (This when the Landing Gear lever is UP or OFF)

Thanks for clarifying!

----------

Nonetheless, that is considered a light indication of the Landing Gear being up and locked. They're not illuminated lights, but still lights with an ON/OFF meaning. Just as the Flight Control "LOW PRESSURE" lights extinguished indicate that the Standby Hyd Pump SOV has been opened after a loss of pressure in A or B flight controls.


Regards,

Alfredo


User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 14, posted (3 years 6 months 3 weeks 6 days 19 hours ago) and read 5098 times:

Quoting bio15 (Reply 13):

Thanks for clarifying!

Anytime....

Quoting bio15 (Reply 13):
Just as the Flight Control "LOW PRESSURE" lights extinguished indicate that the Standby Hyd Pump SOV has been opened after a loss of pressure in A or B flight controls.

Just to add....The Stby system Flt cntrl Low pr light is connected to the Valve position rather than the Low Pressure switch.

regds
MEL.



Think of the brighter side!
User currently offlinebio15 From Colombia, joined Mar 2001, 1089 posts, RR: 7
Reply 15, posted (3 years 6 months 3 weeks 6 days 13 hours ago) and read 5042 times:

Quoting HAWK21M (Reply 14):
Just to add....The Stby system Flt cntrl Low pr light is connected to the Valve position rather than the Low Pressure switch.

The Standby System schematic on the B737NG FCOM shows a direct connection between the Standby Hyd "LOW PRESSURE" and the pressure sensor downstream the Standby Pump, and this connection is broken with the Standby Rudder SOV position (to turn off the Low Press light once the SOV opens). Is there a significant difference between this system and the Classic 737?


Alfredo


User currently offlineHAWK21M From India, joined Jan 2001, 31684 posts, RR: 56
Reply 16, posted (3 years 6 months 2 weeks 3 days 16 hours ago) and read 4604 times:

Quoting bio15 (Reply 15):
Is there a significant difference between this system and the Classic 737

From what I remember offhand....The Low pressure light on the P5 panel for Stby is triggered by valve mvmt.
But will recheck.
regds
MEL.



Think of the brighter side!
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