rampart
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Opening Gear Doors To Cool The Brakes?

Mon Aug 31, 2015 12:59 pm

This was a new one for me.
Flying DEN-PIT the other day. Shortly after takeoff, our pilot announced that we would hold at 20-something thousand feet, while he opened the gear doors. He explained that the brakes were hot (after takeoff??) and needed to be cooled. He said there would be some mild turbulence and loud rushing noise for a few minutes, and there was. Maybe a minute later, it was over and we continued to climb.

How often is this done? And why would brakes still be hot after the aircraft spent time at the gate and then taking off? It was a morning flight, not too hot in the day yet, and it's not as if DEN has short runways requiring lots of braking.

Comments?

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Dalmd88
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RE: Opening Gear Doors To Cool The Brakes?

Mon Aug 31, 2015 1:48 pm

What aircraft type? I don't recall any way to open the gear doors only from the cockpit on any type. The only way is to extend the gear. On the ground they are opened from an access panel near the gear bay.
 
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RE: Opening Gear Doors To Cool The Brakes?

Mon Aug 31, 2015 2:30 pm

I can't think of any modern passenger airliner that allows independent operation of the gear doors.

Given that you had a deal of turbulence and wind noise, I'd submit that the pilot just left the gear down to cool. The brakes are engaged after takeoff to stop the wheels before retracting the gear, hence why they get hot.

[Edited 2015-08-31 07:31:42]
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mmo
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RE: Opening Gear Doors To Cool The Brakes?

Mon Aug 31, 2015 2:33 pm

Quoting rampart (Thread starter):
How often is this done? And why would brakes still be hot after the aircraft spent time at the gate and then taking off? It was a morning flight, not too hot in the day yet, and it's not as if DEN has short runways requiring lots of braking.

Let me guess, it was an Airbus 320 family aircraft!

The 320 has brake cooling fans as an option. The aircraft brakes seem to build up heat very easy, especially with a long taxi. It's been a few years since I've flown the 320 but I think the ECAM message "BRAKE HOT" comes on at 300C and I forget when it goes off (250C sticks in my mind) but with brake fans they do a good job of cooling the brakes. As the previous poster stated, there is no way to open the gear doors, so what the Captain meant to say was he would extend the landing gear. It does happen every now and then, it's not a big deal. Normally if climb performance is not an issue, the gear will be left down a little longer to cool the brakes. Obviously, being in Denver, the decision was made to retract the gear and then extend it above the high terrain. Beats me why operators some operators did not order the brake fans.....
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Woodreau
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RE: Opening Gear Doors To Cool The Brakes?

Mon Aug 31, 2015 3:18 pm

You'd be surprised how warm the brakes stay in flight.

We'd fly a 5 hour flight and the brake temps stay above 150 degrees C during the course of the entire 5 hour flight. Warm enough to keep someone warm in the gear bay if they survive the low oxygen partial pressure.

Once you put the gear down for landing, they cool quite nicely down during the brief time they are down prior to touchdown.
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Brick
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RE: Opening Gear Doors To Cool The Brakes?

Mon Aug 31, 2015 6:02 pm

Back in 2007, I had a flight from BWI on a UA A319 that the flight crew left the gear down after take-off due to a high brake temperature warning. Exactly one week later on another UA A319 flight leaving ANC, the same thing happened. Later when I checked my flight log, both flights were operated by the same aircraft.

This obviously was not a critical issue, but what would be the cause of a specific aircraft constantly experiencing high brake temperatures during taxi?

[Edited 2015-08-31 11:06:28]
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tb727
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 01, 2015 2:38 am

Quoting mmo (Reply 3):
Let me guess, it was an Airbus 320 family aircraft!

Ugh that is one of the most annoying things about the Bus! Have flown a bunch of 319's with them but the 320's don't.

Landing in DEN and doing a fairly quick turn, autobrakes off, max reverse for landing and let it roll, did it just yesterday. LAS has portable brake fans but you could use them almost everywhere at some point in the summer.
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Francoflier
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 01, 2015 5:28 am

Quoting MD95sOverATL (Reply 2):
The brakes are engaged after takeoff to stop the wheels before retracting the gear, hence why they get hot.

Stopping the wheels after takeoff is not what heats the brakes up, it is simply the braking effort required during normal taxi.

Brakes accumulate temperature, which peaks sometime after their use.

Brakes being quite heavy, even in their carbon-ceramic form, and designers tend to try to keep them only as big as strictly necessary.

Airbus' philosophy on brake design is to keep them light and offer brake fans as an option for those users who will be more aggressive on their use (short and frequent sectors).

The A330/340 family suffers from the same issue, where brakes can get pretty hot before takeoff and after landing. Those operators who chose against buying brake fans (mostly long-haul fleets) do sometimes run into scenarios where brakes get hot and takeoffs have to be delayed or gears have to be left extended after takeoff or lowered again later in the flight following a hot gear indication.

Boeing chose to keep its brakes beefier and avoid the complication of a brake fan system and, on their widebodies at least (I don't know about the 737 or 757), brake temperature is almost never an issue. In fact, it is a parameter that is almost never looked at by the pilots.
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flyDTW1992
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 01, 2015 11:19 am

Quoting tb727 (Reply 6):
Landing in DEN and doing a fairly quick turn, autobrakes off, max reverse for landing and let it roll, did it just yesterday. LAS has portable brake fans but you could use them almost everywhere at some point in the summer.

Stupid question...Would a simple solution like a garden hose and a spray nozzle be an option to cool brakes quick at the gate?
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MD95sOverATL
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 01, 2015 12:05 pm

Quoting francoflier (Reply 7):
Stopping the wheels after takeoff is not what heats the brakes up, it is simply the braking effort required during normal taxi.

I stand corrected. Thanks for all the extra info!
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tb727
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 01, 2015 1:59 pm

Quoting flyDTW1992 (Reply 8):
Stupid question...Would a simple solution like a garden hose and a spray nozzle be an option to cool brakes quick at the gate?

No, I think that would be worse, I think it would cool them too quickly and cause some sort of damage/wear/warping. Plus I don't think I would want to breathe in that steam. You want to be cognizant of where the rampers are before you turn the fans on too so you don't blast them with a puff of brake dust when they are trying to chock you. Brake fans aren't supposed to really come on until the temps have peaked but sometimes just prior to the gate guys will turn them on because of this.
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Francoflier
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 01, 2015 3:23 pm

Quoting tb727 (Reply 10):
o, I think that would be worse, I think it would cool them too quickly and cause some sort of damage/wear/warping.

I believe you are right.

I have heard of water being used in extreme overheating cases, but on older generation steel brakes.

I don't think the thermal shock would do the brakes any good, especially if the water was applied unevenly, especially on carbon brakes. Those things have gotten way too insanely expensive to risk playing with them like that...

It's much better to let them air cool, as it happens more gradually and evenly.
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thepinkmachine
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 01, 2015 3:31 pm

It's pretty unusual scenario, even for the A320 - if you get "hot brakes" ECAM during taxi-out, you're not allowed to commence takeoff. Delaying takeoff due to hot brakes is quite common in the Bus. Extending the gear after takeoff - not so.

I had to do this only once. When commencing takeoff the temperature was below limit, but rising, due to a fairly long taxi downhill. We got the caution shortly after liftoff and had to extend the gear. The passengers were reportedly a bit unsettled.

Another scenario to delay gear retraction is takeoff with deactivated brake. Gear is to be left down for a period of time to let the affected wheel spin down...
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 01, 2015 11:36 pm

Is it possible to use reverse idle (like a turboprop would use beta range) during longer downhill taxi to attempt to slow the aircraft some, or is that completely ineffective?
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thegman
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RE: Opening Gear Doors To Cool The Brakes?

Wed Sep 02, 2015 12:58 am

Quoting KC135Hydraulics (Reply 13):

Is it possible to use reverse idle (like a turboprop would use beta range) during longer downhill taxi to attempt to slow the aircraft some, or is that completely ineffective?

They probably wouldn't allow it on a twin. Whereas on a quad you can just take a pair into reverse and slow it down.

Also, I think some airlines taxi out on 1 engine anyways as a fuel savings.
 
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RE: Opening Gear Doors To Cool The Brakes?

Wed Sep 02, 2015 1:16 am

Quoting KC135Hydraulics (Reply 13):

Is it possible to use reverse idle (like a turboprop would use beta range) during longer downhill taxi to attempt to slow the aircraft some, or is that completely ineffective?

It is effective, unfortunately it increases the chance for F.O.D. so isn't done.

On the 727, the only engine approved to be in reverse on taxi was #2 because it directed the air horizontally. Never had to worry about the brakes getting too hot in that thing though, I always did it to boost up 1&3 to get more air flow for the packs to keep the horses cool and the reverse just kept us from going too fast on taxi and it worked well.
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RE: Opening Gear Doors To Cool The Brakes?

Wed Sep 02, 2015 12:56 pm

Quoting KC135Hydraulics (Reply 13):

Is it possible to use reverse idle (like a turboprop would use beta range) during longer downhill taxi to attempt to slow the aircraft some, or is that completely ineffective?

I think most airlines ban the practice (for FOD as already mentioned) but I've seen quite a few biz jets with one or two buckets popped on taxi.
 
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RE: Opening Gear Doors To Cool The Brakes?

Wed Sep 02, 2015 11:06 pm

Quoting N353SK (Reply 16):
I think most airlines ban the practice (for FOD as already mentioned) but I've seen quite a few biz jets with one or two buckets popped on taxi.

I also routinely see CR9s taxiing with 1 reverser deployed. Particularly when it's being taxied by mechanics.
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RE: Opening Gear Doors To Cool The Brakes?

Thu Sep 03, 2015 12:45 am

Quoting flyDTW1992 (Reply 17):
I also routinely see CR9s taxiing with 1 reverser deployed. Particularly when it's being taxied by mechanics.

Which on a CRJ with fuselage mounted engines the risk for fod is greatly reduced.
 
rampart
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RE: Opening Gear Doors To Cool The Brakes?

Thu Sep 03, 2015 2:15 am

Thanks, everyone, for the answers and the education!

Quoting mmo (Reply 3):
Let me guess, it was an Airbus 320 family aircraft!

Indeed it was, and A319 on UA. I forgot to mention the type in my original query.

Quoting MD95sOverATL (Reply 2):
Given that you had a deal of turbulence and wind noise, I'd submit that the pilot just left the gear down to cool.

That makes sense. I thought it would be a more abrupt air speed change if that's the case, but I guess not. It was brief.

Quoting francoflier (Reply 7):
Stopping the wheels after takeoff is not what heats the brakes up, it is simply the braking effort required during normal taxi.

Brakes accumulate temperature, which peaks sometime after their use.

Brakes being quite heavy, even in their carbon-ceramic form, and designers tend to try to keep them only as big as strictly necessary.

Very interesting, thanks!

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RE: Opening Gear Doors To Cool The Brakes?

Thu Sep 03, 2015 3:00 am

Aircraft speed had to be pretty high at FL20 ... Surprised extension is allowed at such higher speeds.

Has anyone else noticed the Buses make a lot more racket and shakes when the gear is extended compared to the 737? Was on a United ship the other day in first that started shaking fairly strong for a few seconds following extension. Felt like some type of resonance. It caught my attention and couldn't recall any similar scenario with a 737.
 
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RE: Opening Gear Doors To Cool The Brakes?

Thu Sep 03, 2015 6:59 am

Quoting mmo (Reply 3):
Let me guess, it was an Airbus 320 family aircraft!

Working in the tower, I have experienced this with many aircraft, right up to the 747SP.

Quoting OKCFlyer (Reply 20):
Aircraft speed had to be pretty high at FL20

The flight level (F200 in this case) is irrelevent if they stay below VLE and VLO...
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RE: Opening Gear Doors To Cool The Brakes?

Thu Sep 03, 2015 7:16 am

Quoting speedbird128 (Reply 21):
Working in the tower, I have experienced this with many aircraft, right up to the 747SP.

I've flown every thing from the 727 (smallest) to the 747-400, 777, 787 and the 320 family is the only aircraft I've ever flown where brake temps were an issue. With the brake fan, it's not an issue at all, but I have operated a private 320 in the UAE without brake fans and it becomes a real issue.

Quoting OKCFlyer (Reply 20):
Aircraft speed had to be pretty high at FL20 ... Surprised extension is allowed at such higher speeds.

Retraction - 220 Extension - 250 Extended - 280 / .67 so those INDICATED airspeeds aren't too high at all. The corresponding MN is really irrelevant and the TAS is also meaningless. It's the indicated airspeed that is the limit speed for extension and retraction.
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rampart
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RE: Opening Gear Doors To Cool The Brakes?

Thu Sep 03, 2015 1:40 pm

Quoting OKCFlyer (Reply 20):

Aircraft speed had to be pretty high at FL20 ... Surprised extension is allowed at such higher speeds.

The pilot did mention that we would slow down to do the gear lowering, and I did feel the deceleration before the noise and vibration commenced.

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RE: Opening Gear Doors To Cool The Brakes?

Thu Sep 03, 2015 4:17 pm

Quoting OKCFlyer (Reply 20):

I believe that the A320 family restricts gear movement above 250 KIAS. Not sure what IAS would be at 20,000 feet on a typical mission but I'm surprised it's not higher.
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KC135Hydraulics
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RE: Opening Gear Doors To Cool The Brakes?

Thu Sep 03, 2015 6:15 pm

The reason that the gear extension and retraction speeds are lower than the extended speeds is due to the landing gear doors. They don't want those ripping off as they open and close.
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thegman
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RE: Opening Gear Doors To Cool The Brakes?

Fri Sep 04, 2015 12:41 am

Quoting DocLightning (Reply 24):
I believe that the A320 family restricts gear movement above 250 KIAS. Not sure what IAS would be at 20,000 feet on a typical mission but I'm surprised it's not higher.

Climbing to final cruise. IDK about the A320, but I would guess 310ish
 
Max Q
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RE: Opening Gear Doors To Cool The Brakes?

Sat Sep 05, 2015 12:47 pm

I agree that this Pilot misspoke when he said he was going to open the gear doors but this is possible in flight.


On the 727 if you placed the gear handle down in flight then immediately placed it to off as soon as the red in transit lights illuminate only the gear doors would open and the gear would stay up.


This was our procedure for a wheel well fire.
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RE: Opening Gear Doors To Cool The Brakes?

Sat Sep 05, 2015 1:49 pm

Quoting mmo (Reply 22):
UAE

The exact place I used to have the SP with hot brakes... Where I currently am the A332 is the one with the regular issues in summer (also no brake fans)... Not the A32X
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RE: Opening Gear Doors To Cool The Brakes?

Sun Sep 06, 2015 4:50 pm

Quoting flyDTW1992 (Reply 8):
garden hose and a spray nozzle be an option to cool brakes

My company's policy is complete brake replacement any time they're sprayed with water or foam in a fire/overheat scenario, for the risks mentioned.

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RE: Opening Gear Doors To Cool The Brakes?

Mon Sep 07, 2015 3:12 pm

What's interesting is that the brakes were hot enough to warrant extending them during flight but not warm enough to delay the take off.  
 
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RE: Opening Gear Doors To Cool The Brakes?

Mon Sep 07, 2015 4:06 pm

Quoting aeroflop (Reply 30):
What's interesting is that the brakes were hot enough to warrant extending them during flight but not warm enough to delay the take off.

That is not that uncommon. After landing and exiting the runway, the brake temp can still only be 100 degC, when you get to the gate, it could be 400 degC. It takes time for the heat to transfer.

Also after liftoff, the brakes are automatically applied to stop the wheels from spinning before retraction.

The A320 series has a sealed gear bay, the brakes stay warm for a long time. At least while taxing the heat can escape to the atmosphere.
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aeroflop
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 08, 2015 3:23 am

Quoting zeke (Reply 31):
The A320 series has a sealed gear bay, the brakes stay warm for a long time. At least while taxing the heat can escape to the atmosphere.

Ahh that makes sense. Any idea why the 320 has a sealed gear bay? Probably more steamline than the open 737 style gear bay.
 
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 08, 2015 5:08 am

Quoting aeroflop (Reply 32):
Ahh that makes sense. Any idea why the 320 has a sealed gear bay? Probably more steamline than the open 737 style gear bay.

The B737 was designed for short flights and cheap fuel. The additional doors would add weight, complexity and cost, which would have to be balanced against extra drag omitting them causes. Almost certainly Boeing would not make the same decision today, given that range has doubled or more, and fuel costs have gone through the roof, but changing the existing design is going to be near impossible (this is similar to the problem of lengthening the main gear on the 737 - there just isn't any room in that area to do anything like that).

The A320 was launched (20 years later) with substantially better range that the 731, and with much higher fuel costs. The advantages of "proper" gear doors would have been much larger.

And it's still a choice you could make - some of the smaller, shorter legged, Citations, for example, omit gear doors as well.
 
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HAWK21M
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RE: Opening Gear Doors To Cool The Brakes?

Sun Sep 20, 2015 9:16 am

Is there any Aircraft type that permits the gear door opening in flight without extending the gear......anyone aware.
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RE: Opening Gear Doors To Cool The Brakes?

Mon Sep 21, 2015 5:22 am

Quoting mmo (Reply 3):
Beats me why operators some operators did not order the brake fans.....

Cost, weight, and MX man hours (those have to be partially disassembled for every tire and brake change) would be my best guess. Just shooting from the hip there, but that seems like the normally reasons airlines forego things.

Quoting KC135Hydraulics (Reply 13):

Is it possible to use reverse idle (like a turboprop would use beta range) during longer downhill taxi to attempt to slow the aircraft some, or is that completely ineffective?

I don't know what their SOP is but I did see an AA 319 doing just that heading over to the 25s at LAX about a month back. Taxiway Bravo does indeed have a mile or two of slight down-hill so I wasn't exactly surprised to see this, but I have to admit I've never seen an airliner do that prior.
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Max Q
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RE: Opening Gear Doors To Cool The Brakes?

Mon Sep 21, 2015 7:19 am

Quoting HAWK21M (Reply 34):
Is there any Aircraft type that permits the gear door opening in flight without extending the gear......anyone aware.

Yes, the B727.


It is the procedure for a wheel well fire.


(See post 27)
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KC135Hydraulics
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RE: Opening Gear Doors To Cool The Brakes?

Mon Sep 21, 2015 11:52 pm

As was the reading earlier, any aircraft whose aircraft has a landing gear OFF position will probably be able to open the doors without the gear coming down. You just need to beat the gear doors with the handle, IE select gear DOWN and quickly to OFF, or the gear will continue to receive hydraulic pressure and unlock/extend once the doors are fully open (sequence valve stuff).
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CALTECH
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 22, 2015 12:47 am

Quoting Max Q (Reply 27):
On the 727 if you placed the gear handle down in flight then immediately placed it to off as soon as the red in transit lights illuminate only the gear doors would open and the gear would stay up.
This was our procedure for a wheel well fire

Thought it was by either just lowering the gear, or lowering the gear with the gear handle in the down position, then move the gear handle momentarily to the up position which started the door opening sequence. Then place the handle in OFF when the gear door light came on. That will leave the doors open and the landing gear down to help with the fire. Thought that was the procedure for a wheel well fire. Didn't want that gear in the wheel well if the fire warning went off. Did those checks a lot during gear swings on the 27. Could be off a bit though, that was so long ago.
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KC135Hydraulics
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 22, 2015 3:10 am

Quoting CALTECH (Reply 38):
Thought it was by either just lowering the gear, or lowering the gear with the gear handle in the down position, then move the gear handle momentarily to the up position which started the door opening sequence. Then place the handle in OFF when the gear door light came on. That will leave the doors open and the landing gear down to help with the fire. Thought that was the procedure for a wheel well fire. Didn't want that gear in the wheel well if the fire warning went off. Did those checks a lot during gear swings on the 27. Could be off a bit though, that was so long ago.

That would seem to make more sense, to leave the gear out of the bay (exploding tires in a wheel well can't be good). In that scenario, the procedure you described would have the desired effect of leaving both the gear and the doors down.
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mmo
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 22, 2015 6:06 am

Quoting KC135Hydraulics (Reply 37):

As was the reading earlier, any aircraft whose aircraft has a landing gear OFF position will probably be able to open the doors without the gear coming down. You just need to beat the gear doors with the handle, IE select gear DOWN and quickly to OFF, or the gear will continue to receive hydraulic pressure and unlock/extend once the doors are fully open (sequence valve stuff).

I can assure you if you were to do that in any aircraft in commercial and it wasn't in the QRH, you would be having a nice one to one chat with the Chief Pilot. The 727 is the only Boeing aircraft I've flown where that is in the QRH. Every other Boeing I've flown (757,747 (all models), 777,787), the procedure is to lower the gear and nothing else. The days of "troubleshooting" are long gone.
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Max Q
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RE: Opening Gear Doors To Cool The Brakes?

Tue Sep 22, 2015 9:33 am

Quoting CALTECH (Reply 38):
Thought it was by either just lowering the gear, or lowering the gear with the gear handle in the down position, then move the gear handle momentarily to the up position which started the door opening sequence. Then place the handle in OFF when the gear door light came on

No, the procedure was:


Gear handle down.


As soon as the red gear in transit lights illuminate.


Gear handle off.


This opens the gear doors and leaves the gear retracted.



Your description is inaccurate.
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CALTECH
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RE: Opening Gear Doors To Cool The Brakes?

Wed Sep 23, 2015 7:29 pm

Quoting Max Q (Reply 41):
This opens the gear doors and leaves the gear retracted.

Have to look it up. Doesn't seem right with a wheel well fire warning that you would leave the gear retracted. IIRC, you were supposed to extend the gear to cool off the brakes.
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Max Q
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RE: Opening Gear Doors To Cool The Brakes?

Thu Sep 24, 2015 5:15 am

Looking at my B727 flight manual I reviewed the procedure for wheel well fire.


Airspeed 270 KIAS / .83M MAX.


Landing gear down.


Landing gear handle UP, THEN OFF.


Note, Landing gear handle must remain in the UP position until gear door lights (forward and S/O panel illuminate.


After landing gear is extended momentarily position the gear handle to the UP position and immediately to the OFF position AFTER gear door lights illuminate indicate opening of gear doors.


If wheel well fire warning extinguishes:


Allow a minimum of 20 minutes cooling prior to retracting landing gear. Observe maximum gear retraction speeds.

OR:


If wheel well fire warning remains illuminated:


Land at nearest airport.



So yes, the procedure details how to extend the gear and leave the gear doors open in an attempt to extinguish the fire, my memories not what it used to be.
The best contribution to safety is a competent Pilot.


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CALTECH
Posts: 3296
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RE: Opening Gear Doors To Cool The Brakes?

Thu Sep 24, 2015 12:09 pm

Quoting Max Q (Reply 43):
So yes, the procedure details how to extend the gear and leave the gear doors open in an attempt to extinguish the fire, my memories not what it used to be.

My memory is not what it used to be either. Forgetting things from long ago, until something stirs those unused neurons into firing. Cut my teeth on the 727-100/-200 and the DC-10-10/-30s. Miss that 727.......
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Max Q
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RE: Opening Gear Doors To Cool The Brakes?

Thu Sep 24, 2015 6:37 pm

Quoting CALTECH (Reply 44):
My memory is not what it used to be either. Forgetting things from long ago, until something stirs those unused neurons into firing. Cut my teeth on the 727-100/-200 and the DC-10-10/-30s. Miss that 727.......

It was and is a great aircraft.


Great to see the # 1 727-100 being restored for flight, looks good in the old UAL colors but I miss the 'Golden tails'
The best contribution to safety is a competent Pilot.


Guns are a malignant cancer that are destroying our society
 
Airstud
Posts: 4729
Joined: Wed Nov 29, 2000 11:57 am

RE: Opening Gear Doors To Cool The Brakes?

Thu Oct 01, 2015 2:56 am

Quoting MD95sOverATL (Reply 2):
The brakes are engaged after takeoff to stop the wheels before retracting the gear

Interesting... but why? What's the harm from just letting them spin till they stop?         
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Francoflier
Posts: 4935
Joined: Wed Oct 31, 2001 12:27 pm

RE: Opening Gear Doors To Cool The Brakes?

Thu Oct 01, 2015 3:13 am

Quoting Airstud (Reply 46):
What's the harm from just letting them spin till they stop?

The gyroscopic torque induced by angling these heavy spinning wheels towards the horizontal would induce high and potentially damaging mechanical loads on the gear assembly.

Nose wheels are not braked before retraction as they remain at the same angle when they go up, which avoids having to install some sort of braking device there. However they are often 'snubbed' by friction pads once they reach the stowed position in the gear bay to avoid prolonged vibrations and noises after take-off.

Which reminds me that I've always wondered how they dealt with that sideways retracting nose gear on the Trident...
  

[Edited 2015-09-30 20:14:20]
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CALTECH
Posts: 3296
Joined: Thu May 17, 2007 4:21 am

RE: Opening Gear Doors To Cool The Brakes?

Thu Oct 01, 2015 3:45 am

Quoting Airstud (Reply 46):
Interesting... but why? What's the harm from just letting them spin till they stop?         

Any unbalance of the wheels results in vibration to the aircraft structure, and. in the case of retractable landing gears, the rotation provides a source of possible damage to the wheel well when the gear is retracted. If the tire thread lets go, you have a hunk of rubber flying around in the wheel well. With all the hydraulics and other systems running through the wheel well, would be a most undesirable effect.

http://cdn-www.airliners.net/aviation-photos/photos/1/8/8/1705881.jpg
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twal1011727
Posts: 449
Joined: Thu Mar 23, 2006 5:36 am

RE: Opening Gear Doors To Cool The Brakes?

Thu Oct 01, 2015 7:12 pm

Quoting Airstud (Reply 46):
Interesting... but why? What's the harm from just letting them spin till they stop?

Watch top-fuel or funny car racing. See how tall the tires grow to when they burn out the tires just before racing.
While not as extreme, the aircraft tires do expand and with the tight openings in the wheel wells it could be a problem.

www.youtube.com/watch?v=-VF0JwxQqcA

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