47Chits

Flew Seattle to Phoenix, weather was pretty normal in my opinion (not sweltering hot).

Shortly after takeoff (5 mins, maybe 20000 feet) the captain announces that the brakes are overheated and we need to lower the landing gear and fly like that for a few minutes, and this happens "all the time".

Never had it happen before, was interesting (and noisy!).

How does this happen on takeoff? And after thousands of flights, this is a first for me.

Trying to understand how this works is all.

onenorth

Aircraft type?

The brakes are used to stop wheel rotation after takeoff. Also the pilot may have been riding the brakes during taxi. Once retracted they may not cool down.

47Chits

Makes sense. Was a A321.

QUOTE=onenorth;35530746]Aircraft type?

The brakes are used to stop wheel rotation after takeoff. Also the pilot may have been riding the brakes during taxi. Once retracted they may not cool down.[/QUOTE]

fdog

Brake cooling is a big thing. After using them for landing, they take longer than you'd think to cool off.

For example, SouthWest has a reputation for landing fast and taxiing quickly. All this heats up the brakes; you may have noticed that SW has no main gear doors over the wheels on their 73's. Thus, in flight, they cool down for the next fast landing.

I seem to recall when SW ordered new aircraft back in 2000, they selected steel rotors instead of carbon fiber, in spite of an 800 lb penalty. The steel rotors cooled faster than the carbon fiber. I believe that SW's rotor choices have since changed, but it was interesting how much emphasis was placed on brake cooling.

All the best, James

Antarius

No one has main gear doors on their 737s as the 737 doesn't have them as part of their design.

zkzkz

Those sound not serious and more likely. Another possibility -- which would be serious and given what the pilot said doesn't sound likely -- and on a A320 is probably unlikely anyways but used to be a common issue on smaller regional planes is brakes dragging even when not applied. If not caught that could lead to insufficient speed on takeoff or overheating or fire. I assume major airliners don't have issues like that any more or can detect it reliably.

nachosdelux

brakes remain hot after previous landing. A long taxi prior to takeoff would exacerbate the situation. The brakes (or snubbers) used to stop rotation at retraction would not be an issue.

47Chits

The plane was parked at the gate for about an hour an a half. I can see the landing heating them up like crazy, but a takeoff just seemed like it was a different issue. Never had this issue and the captain's comment about it being common was interesting as I have never had to fly so high with the landing gear down for a few minutes. Maybe also they weren't adjusted properly and just rubbing on the wheels. Wasn't a particularly hot evening in Seattle when we took off. All is good, was an interesting experience I had never had before.

IADCAflyer

No 737s have gear doors - at least not on the main landing gear.

JJeffrey

Never seen nor heard of that either in lots of years of flying.

I suspect that the captain saying "it happens all the time" was just a slight exaggeration to minimize any concern from the pax.

tecate55

Trying to find the chart that I saw years ago, but basically brakes don't cool down very fast while sitting on the ground. Can take a few hours to cool back down to atmospheric temp sitting on the ground. On enough short hops/short turns, the heat accumulates on you. And you said evening, so this is quite possible.

From a Boeing doc here: https://www.smartcockpit.com/docs/Br..._Operation.pdf :

"brake energy can become a problem from a series of closely-spaced routine stops just as much as a single high energy stop, due to the inherently slow nature of brake cooling"

"Inflight Inflight Gear Retracted Cooling: In the best case, approximately equivalent to on ground cooling in still air, For some models, only about 1/3 as effective as ground cooling"

"Delayed retraction after takeoff - If performance limited, gear may need to be retracted normally for terrain clearance, and then re-extended when safe altitude is reached"

"Inflight gear extended cooling is about ten times more effective than still air ground cooling!"

Found the graph here!

https://aviation.stackexchange.com/q...ypical-landing

Now, why would they takeoff if the brakes were hot? My guess is that they weren't hot enough to delay takeoff, but the next turn was going to be a short(er) one. And better to lower the gear after takeoff so you don't run the risk of lowering them too late closer to landing and needing to work it out with ATC or delay the last turn of the day.

Why the retract and deploy? Maybe fewer minutes of deployment required at higher and colder altitudes (but more speed = more drag, but thinner air = lower drag and lower cooling...)? Or just followed the standard procedures for takeoff before doing the "nice to have" favour.

When you're cooling something that's a few hundred degrees celsius, it doesn't matter much whether ambient is 20C or 40C. A 40C breeze will cool better than 20C still air.

Not a pilot, but have thought about cooling stuff a lot.

VegasGambler

My last flight (BUR-PHX) was actually delayed slightly because the brakes were too hot. The captain said something about the runway at BUR being short and this being fairly common as a result of them having to brake a little harder when they land, so we had to wait for them to cool down. We ended up waiting about 15 minutes or so, and still arrived "on time" (there is definitely some buffer time built into the scheduled times so that's not surprising -- we basically landed at the scheduled time instead of 15 minutes early like we usually would)

Anyway, the point is, if the brakes are too hot from the prior landing, they won't be able to take off. So this does not sound like a case of that happening.

ACES70

happens fairly regularly when watching you tube - they keep the wheels down either because brakes are hot or the wheels are still spinning.

PHLGovFlyer

[Caution - Nerd Alert]

Technically all 737s do have main landing gear doors, they just don't cover/enclose the wheels and tires. A good description of the system and why it was designed that way is from about 10:02 to 15:40 in this video:


Doors like the ones on the 737 main landing gear are often referred to as "Strut Doors". These types of main landing gear doors cover/enclose the struts when the gear is retracted. A good image/description is at about 13:33 in the linked video.

onenorth

Yes, my understanding is that takeoff is prohibited if the brakes are too hot. So whatever caused the overheating alarm occurred after takeoff.