Landing gear, (passenger planes.)
Discussion
Apologies if it has been asked before, but something i'm just curious about.
When i've watched a passenger plane take off from an airport, (normally Heathrow) it seems that pretty much as the plane has left the runway the wheels come up very quickly.
Yet when i'm in central London i often look up at the planes on approach to Heathrow and the wheels are already down.
Are there any specifics as to why these 2 things happen this way?
TIA.
When i've watched a passenger plane take off from an airport, (normally Heathrow) it seems that pretty much as the plane has left the runway the wheels come up very quickly.
Yet when i'm in central London i often look up at the planes on approach to Heathrow and the wheels are already down.
Are there any specifics as to why these 2 things happen this way?
TIA.
chilistrucker said:
Apologies if it has been asked before, but something i'm just curious about.
When i've watched a passenger plane take off from an airport, (normally Heathrow) it seems that pretty much as the plane has left the runway the wheels come up very quickly.
Yet when i'm in central London i often look up at the planes on approach to Heathrow and the wheels are already down.
Are there any specifics as to why these 2 things happen this way?
TIA.
Gear down varies dependant upon aircraft type, adn the companies Standard Operating Procedures (SOP's). When i've watched a passenger plane take off from an airport, (normally Heathrow) it seems that pretty much as the plane has left the runway the wheels come up very quickly.
Yet when i'm in central London i often look up at the planes on approach to Heathrow and the wheels are already down.
Are there any specifics as to why these 2 things happen this way?
TIA.
some aircraft are difficult to slow, (a330 series is an example) so dropping the gear helps the pilot slow the aircraft to the speed ATC have asked. LHR fly 160knt to 4nm from the threshold and too fast or slow, causes problesm with goarounds, which impacts on the landing rate.
From my office i can tell you that gear up is far more regimented and will revolve around airspeed and climb rate.
A pilot will be along shortly to rubbish all the above.
Landing gear will have a limiting speed (both upon travel and locked down speed).
Generally on T/O you have full (or at least a high) power setting set so the a/c is accelerating quickly, ergo you get the gear up as soon as you can.
Couple this with the fact that the gear produces LOTS of drag so, in the case of an EFATO (engine failure after t/o) you also want the gear up.
Generally on T/O you have full (or at least a high) power setting set so the a/c is accelerating quickly, ergo you get the gear up as soon as you can.
Couple this with the fact that the gear produces LOTS of drag so, in the case of an EFATO (engine failure after t/o) you also want the gear up.
When on the approach it gets quite busy so putting the gear down "early" also helps reduce workload. Baby pilots start off with putting their gear down while on the downwind part of the circuit, which is very early, relatively speaking. You also aren't getting any quicker so there's no risk of ripping off the gear.
On take-off once you have a positive rate of climb the workload is starting to reduce (ish - airspace and radio work can be a bit busy, but airliners have someone else to do that :-)). You are also accelerating and it's better to get things out of the way rather than discover you forget to retract the flaps and gear when you are in the cruise ... and you've left bits of them behind.
On take-off once you have a positive rate of climb the workload is starting to reduce (ish - airspace and radio work can be a bit busy, but airliners have someone else to do that :-)). You are also accelerating and it's better to get things out of the way rather than discover you forget to retract the flaps and gear when you are in the cruise ... and you've left bits of them behind.
When flying with my old man he puts the gear up when There's a positive rate of climb same with the flaps usually around 200ft
When landing it creates drag which slows the aircraft so again I've seen him put the gear down on the base leg (just before finals)
In the event of a go around again the wheels come up when there's a positive rate of climb
When landing it creates drag which slows the aircraft so again I've seen him put the gear down on the base leg (just before finals)
In the event of a go around again the wheels come up when there's a positive rate of climb
There is also the not entirely un-important factor of what happens if the gear fails in any way!
On take off, as soon as you have sufficient climb rate, you stow the gear. Should it fail to retract (fully or partially) no probs, because you already had a positive climb rate.
But on landing, if you waited till the last second to drop the gear, and it malfunctioned, you make your go-around a lot more fraught than it would have been if you found out the gear was foo-barr'd during the earlier stages of the descent!
On take off, as soon as you have sufficient climb rate, you stow the gear. Should it fail to retract (fully or partially) no probs, because you already had a positive climb rate.
But on landing, if you waited till the last second to drop the gear, and it malfunctioned, you make your go-around a lot more fraught than it would have been if you found out the gear was foo-barr'd during the earlier stages of the descent!
truck71 said:
Interesting, heading East along the M4 on Friday circa 1330 there was a BA flight heading North West that was probably a couple of mins into flight just West of Windsor with the gear down. Very odd.
The next flight to depart was already airborne and heading South West, gear up.
Sometimes there is a technical reason you have to leave the gear down after take off, obviously if it's stuck or even some brake issues mean you don't raise the gear until a certain height. Or perhaps they forgot. The next flight to depart was already airborne and heading South West, gear up.
Most airlines specify their crews have their aircraft stabilised in the landing configuration by a certain height. (Landing flaps and gear down) this avoids last minute heroics or too much individual expression from the pilots near the ground.
Air traffic control (in most countries) will want aircraft flying the same speeds on approach for planning purposes. This might typically be 160 knots until 4 miles from landing. This wil result in different flaps settings on different aircraft or even the same aircraft at different weights.
So say you have to have landing flaps and gear down by 1000ft depending on the aircraft type, you might need to be selecting gear down somewhere around 2000ft above the ground. Isometimes it's hard to slow down some aircraft ( to fly the speeds air traffic control want) if there is a tail wind or they are light so they might take the gear down early and use it as a brake.
I agree with the consideration to gear proproblems. I've had probs on the jumbo and 737. Both took a little time to resolve but neither required a go-around.
If the departure with gear still down was an Airbus, they may have been cooling the brakes a little. It isn't a standard procedure but is fairly effective. The links are designed to reduce likelihood o f combustion if fluids leak and drip into hot brakes in the wheel well.
If the departure with gear still down was an Airbus, they may have been cooling the brakes a little. It isn't a standard procedure but is fairly effective. The links are designed to reduce likelihood o f combustion if fluids leak and drip into hot brakes in the wheel well.
Mabbs9 said:
If the departure with gear still down was an Airbus, they may have been cooling the brakes a little. It isn't a standard procedure but is fairly effective. The links are designed to reduce likelihood o f combustion if fluids leak and drip into hot brakes in the wheel well.
How hot can the brakes get from taxiing from the gate to the runway? Aircraft never seem to go very fast and it always feels very gentle. Are Airbus worse for this than other manufacturers?peter tdci said:
How hot can the brakes get from taxiing from the gate to the runway? Aircraft never seem to go very fast and it always feels very gentle. Are Airbus worse for this than other manufacturers?
Not sure of temps, but Emirates have trouble cooling the brakes on the A380 for flights with 2hr turnarounds. LHRFlightman said:
Not sure of temps, but Emirates have trouble cooling the brakes on the A380 for flights with 2hr turnarounds.
Ah, so it's not the taxi out that warms them up, they are still trying to get them cool from the previous landing?It would seem easier to get some air blowers installed at the gates, rather than dangling the gear out after take-off. Cheaper too I would suspect given the drag and corresponding fuel burn?
peter tdci said:
How hot can the brakes get from taxiing from the gate to the runway? Aircraft never seem to go very fast and it always feels very gentle. Are Airbus worse for this than other manufacturers?
Usually the cumulative effect of previous landing then the taxy. Most are fitted with fans to help keep the temps down. The limit is 300 degrees to takeoff. After landing on a short runway it's easy to be well over 400.We can mitigate this somewhat by delaying the start of the second engine and likewise shutting one down after landing.
Atb
As you rotate and lift the aircraft into the air, with a positive rate of climb the gear is raised (to help with climb performance).
As you land most airlines these day have a stabilisation criteria which means gear down and final flap setting by around 1500 feet (around 5 miles out) - some people drop the gear earlier than that too.
Hope it helps and no question is a stupid question.
As you land most airlines these day have a stabilisation criteria which means gear down and final flap setting by around 1500 feet (around 5 miles out) - some people drop the gear earlier than that too.
Hope it helps and no question is a stupid question.
jinkster said:
As you rotate and lift the aircraft into the air, with a positive rate of climb the gear is raised (to help with climb performance).
As you land most airlines these day have a stabilisation criteria which means gear down and final flap setting by around 1500 feet (around 5 miles out) - some people drop the gear earlier than that too.
Hope it helps and no question is a stupid question.
I'm glad i asked, as a lot of info posted here that i knew nothing about as its an industry i'm not involved in.As you land most airlines these day have a stabilisation criteria which means gear down and final flap setting by around 1500 feet (around 5 miles out) - some people drop the gear earlier than that too.
Hope it helps and no question is a stupid question.
Right, who's going to give me a go in their jumbo?
Mabbs9 said:
peter tdci said:
How hot can the brakes get from taxiing from the gate to the runway? Aircraft never seem to go very fast and it always feels very gentle. Are Airbus worse for this than other manufacturers?
Usually the cumulative effect of previous landing then the taxy. Most are fitted with fans to help keep the temps down. The limit is 300 degrees to takeoff. After landing on a short runway it's easy to be well over 400.We can mitigate this somewhat by delaying the start of the second engine and likewise shutting one down after landing.
Atb
The brake packs on airliners are constrained to relatively small diameter discs, but are made up from multiple discs. There are massive multi-piston calipers around most of the circumference of the discs and there are pads both either side like a car but protruding between each of the discs too - so it's a big Scooby Snack multi-layered sandwich of brake disc and pads to give the swept area needed to effect the braking force needed. These heat up royally on landing or perhaps worse a rejected take-off (where the aircraft is at its heaviest / hardest to stop) and because so little of the brakes is exposed to fresh air they can take ages to cool down even with forced cooling fan assemblies attached which blow air over them whilst parked on the ground for the offload/reload).
Vaguely on-topic, a swissair caravalle crashed as a result of a brake fire in 1963.
It is thought the crew decided to try to clear runway fog by taxiing at high thrust and holding it on the brakes.
https://en.wikipedia.org/wiki/Swissair_Flight_306
It is thought the crew decided to try to clear runway fog by taxiing at high thrust and holding it on the brakes.
https://en.wikipedia.org/wiki/Swissair_Flight_306
Thing to remember about brakes. Conservation of Energy.
"Energy can neither be created nor destroyed; rather, it transforms from one form to another. "
In other words, kinetic energy (movement) is transferred (reduced) into heat energy by the friction of the pads against the moving disc. The heat energy is then transferred into the surrounding air (it tries to heat up the air around it).
Reference the aircraft I fly.
Stopping an aircraft at 351,000kg at a speed of 180kts at sea level 15*C requires 76.8 million foot pounds of energy to be dissipated. That's a lot of heat and that will cause the fuse plugs to melt and the tire will deflate.
Even during taxi at slower speeds application of brakes to prevent acceleration puts energy into the brakes. This is cumulative in that for every taxi mile is calculated as 1.0 million foot pounds. Long taxi, downhill, light aircraft high elevation it all adds up.
So basically brake fade until your wheels, tires, brake fluid, dust and all that jazz catch fire.
Off for a lie down now.
"Energy can neither be created nor destroyed; rather, it transforms from one form to another. "
In other words, kinetic energy (movement) is transferred (reduced) into heat energy by the friction of the pads against the moving disc. The heat energy is then transferred into the surrounding air (it tries to heat up the air around it).
Reference the aircraft I fly.
Stopping an aircraft at 351,000kg at a speed of 180kts at sea level 15*C requires 76.8 million foot pounds of energy to be dissipated. That's a lot of heat and that will cause the fuse plugs to melt and the tire will deflate.
Even during taxi at slower speeds application of brakes to prevent acceleration puts energy into the brakes. This is cumulative in that for every taxi mile is calculated as 1.0 million foot pounds. Long taxi, downhill, light aircraft high elevation it all adds up.
So basically brake fade until your wheels, tires, brake fluid, dust and all that jazz catch fire.
Off for a lie down now.
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