Spark
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Take Offs In The Rain?

Thu Oct 11, 2007 4:57 am

A while back there was a discussion about long take off rolls, mentioning planes taking off in the rain. I'm a hobbyist, and hadn't noticed the difference before (in my defense, I live in California, and it doesn't rain here- ok, it doesn't rain much, and it is usually a light rain).
Well, on Sunday I flew from PDX to OAK in a 734. We took off in a fairly heavy rain (west coast standards), and plane climbed heavier than usual. I've looked back, and it is my first time taking off in the rain for a very long time (maybe 20 years).
My question is: how much difference is there in the behavior of an airplane during a rain storm versus the typical Northern California fall weather (you know that really terrible weather we get during our fall- it almost 70 F, and there might be 2 or 3 clouds disrupting our skyline).
 
ConcordeBoy
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RE: Take Offs In The Rain?

Thu Oct 11, 2007 5:03 am

On one hand, rain can actually enhance takeoff power, as light amounts of moisture have the effect of increasing intake density.

On the other hand, the takeoff has to be recalculated with the notion of a slicker runway and other atmospheric phenomena in mind.
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SkyexRamper
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RE: Take Offs In The Rain?

Thu Oct 11, 2007 5:40 am

Quoting Spark (Thread starter):
We took off in a fairly heavy rain (west coast standards), and plane climbed heavier than usual. I've looked back, and it is my first time taking off in the rain for a very long time (maybe 20 years).

By heavier do you mean at a low pitch angle? Sounds like the pilots didn't want to ingest large amounts of rain water in the climb.
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phxpilot
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RE: Take Offs In The Rain?

Thu Oct 11, 2007 9:52 am

Quoting Skyexramper (Reply 2):
By heavier do you mean at a low pitch angle? Sounds like the pilots didn't want to ingest large amounts of rain water in the climb.

Not true. On takeoff you pitch to the command bars on the flight director. No adjustment is made for precipitation. The variable is how quickly you effect the pitch change on rotation, but that should not vary perceptibly.

It is also very difficult for a layperson to judge from the passenger seat exactly what is going on with the command and performance of the aircraft. There are simply too many variables and too little perspective from a pax point of view to make a judgement as to a "heavier climb."

Weight, elevation, temperature, pressure, runway, thrust, etc., all figure into the takeoff, rotation and climb performance of an aircraft. Any one of these variables can make a takeoff and climb seem lighter or heavier.
 
Geo772
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RE: Take Offs In The Rain?

Thu Oct 11, 2007 10:26 am

On shorter runways in the rain (compared to dry) the takeoff roll is likely to be quicker because you have to allow for more runway to stop on in the event of a rejected takeoff.
But as has been said you'd be hard pressed to notice the difference in the cabin, unless you do the trip often and you have a stopwatch.
Personally I rather like leaving in the wet, as I know in only a few minutes it will be sunny again.  Smile
Flown on A300B4/600,A319/20/21,A332/3,A343,B727,B732/3/4/5/6/7/8,B741/2/4,B752/3,B762/3,B772/3,DC10,L1011-200,VC10,MD80,
 
Norcal773
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RE: Take Offs In The Rain?

Thu Oct 11, 2007 10:50 am

Quoting Spark (Thread starter):
I live in California, and it doesn't rain here- ok, it doesn't rain much, and it is usually a light rain).



Quoting Spark (Thread starter):
(you know that really terrible weather we get during our fall- it almost 70 F, and there might be 2 or 3 clouds disrupting our skyline).

What part of California are you from??  Yeah sure
If you're going through hell, keep going
 
mark5388916
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RE: Take Offs In The Rain?

Thu Oct 11, 2007 1:34 pm

Quoting Norcal773 (Reply 5):

Must be near mine, in Souther Calfornia, but its 6:30 at the moment and its 45 or so.

Mark
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747fan
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RE: Take Offs In The Rain?

Thu Oct 11, 2007 2:41 pm

Quoting ConcordeBoy (Reply 1):
On the other hand, the takeoff has to be recalculated with the notion of a slicker runway and other atmospheric phenomena in mind.

 checkmark  I believe that if the runway is very wet (especially if there's standing water/ponding), pilots are required to perform a "max-power takeoff" rather than using a reduced thrust setting (most takeoffs are not made at max power) to save engine wear (set based on variables such as rwy. length, pax/fuel load, weather conditions, etc.). I'm assuming they have to do this so they could have more runway ahead of them if they had to abort the takeoff near V1, as stopping distances are longer on a wet surface.
I doubt precipitation has much of an effect on climb rate. I do think that sometimes a wet surface can lead to a slightly longer TO run, correct me if I'm wrong.
 
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Starlionblue
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RE: Take Offs In The Rain?

Thu Oct 11, 2007 2:48 pm

Quoting 747fan (Reply 7):
I do think that sometimes a wet surface can lead to a slightly longer TO run, correct me if I'm wrong.

Wrong. Don't make me point you to the conveyor belt thread.  Wink
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
bond007
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RE: Take Offs In The Rain?

Thu Oct 11, 2007 3:28 pm

Quoting Starlionblue (Reply 8):
Wrong. Don't make me point you to the conveyor belt thread.

LOL ... well only true if the speed of the rain is matching the speed of the wheels  Smile


Jimbo
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pilotaydin
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RE: Take Offs In The Rain?

Thu Oct 11, 2007 4:31 pm

there may be slower acceleration if the runway surface is not properly constructed, and V1 is usually set back about 10 knots ball park in the 737, but the actual climb, the only thing that changes is the end of runway screen height drops from 35 feet to 15 in some cases and 0 in others  Smile
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747fan
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 1:50 am

Quoting Starlionblue (Reply 8):
Wrong. Don't make me point you to the conveyor belt thread.

Yeah, I don't feel like overtaxing my computer's hard drive right now...  Wink
Thanks for correcting me, as it appears I was indeed wrong; I was basing what I said on an observation while spotting during the rain.
My question is this, aren't pilots required to use max takeoff thrust if the runway is contaminated (such as with standing water)?
 
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seabosdca
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 2:12 am

Quoting 747fan (Reply 7):
believe that if the runway is very wet (especially if there's standing water/ponding), pilots are required to perform a "max-power takeoff" rather than using a reduced thrust setting

The last takeoff I experienced that felt from a pax perspective like full power was in an AS 738 from BOS in heavy snow at night. There was a thin coating of snow on the runway and I expect this would have considerably affected stopping distance in case of problems before V1.

The aircraft was not full (although it would have had lots of fuel aboard for the 6+ hour transcon) and the takeoff was most impressive, especially as we gained speed and the landing lights illuminated what looked like constantly moving shiny streaks against the blackness...
 
pilotboi
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 2:20 am

Quoting 747fan (Reply 7):
I do think that sometimes a wet surface can lead to a slightly longer TO run, correct me if I'm wrong.



Quoting Starlionblue (Reply 8):
Wrong. Don't make me point you to the conveyor belt thread.



Quoting Pilotaydin (Reply 10):
there may be slower acceleration if the runway surface is not properly constructed

He's not necessarily wrong. It's not a wet surface that will cause a longer takeoff, but water in general, or any contamination (snow, dirt, debris, etc.). It causes a slower acceleration which in turn causes a longe takeoff roll.

But of course this is a plus for landing!
 
Spark
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 2:32 am

Thank you for your responses, they were helpful.
Like you may have noticed, I'm a total novice when it comes to the actual dynamics of a flight- definitely more aware of what is happening because I play with FS and am interested in stuff like that, but don't know why things happen. Now, if we were talking about Special Education, teaching, or Algebra and I could actually make some inteligent comments (I don't think there are a lot of Special Education and Algebra hobbyist out there).

When I said a heavy climb, I meant that the climb seemed to be slower than usual (but it may have been my imagination) but the take off wasn't unusual. The pilot probably was trying to keep from sucking up water (it was a good NW rain shower).

BTW, I live about 10 minutes south of OAK (in Hayward).
 
PhilSquares
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 5:39 am

Quoting 747fan (Reply 7):
I believe that if the runway is very wet (especially if there's standing water/ponding), pilots are required to perform a "max-power takeoff" rather than using a reduced thrust setting (most takeoffs are not made at max power) to save engine wear (set based on variables such as rwy. length, pax/fuel load, weather conditions, etc.).

On any aircraft I've flown there is no requirement to do a max thrust take off because of wet runway. However, there are some MEL items that can dictate that, for instance anti-skid inop. In fact, on a contaminated runway, there are some aircraft that actually reduce thrust to increase performance with respect to VMCG.

Quoting 747fan (Reply 7):
I doubt precipitation has much of an effect on climb rate. I do think that sometimes a wet surface can lead to a slightly longer TO run, correct me if I'm wrong.

There is no performance adjustment for take off roll due to wet runways.
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Qantas744er
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 6:12 am

Quoting 747fan (Reply 7):
I doubt precipitation has much of an effect on climb rate. I do think that sometimes a wet surface can lead to a slightly longer TO run, correct me if I'm wrong.

Well high amounts of water going into the engine can lead to a surge and for me that can affect climb rate  Smile  Silly so on many aircraft CON ignition is turned on such as on the 744 so that the ignition will immediatly re-ignite the engine. Very good feature and many airline SOP's such as CX require this to be turned on if their 744 and all toher planes that have the feature when its raining hard.

Leo
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pilotaydin
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 8:30 am

Quoting 747fan (Reply 11):
My question is this, aren't pilots required to use max takeoff thrust if the runway is contaminated (such as with standing water)?

on the 737-400, we don't use assumed thrust for a wet runway, but on the 737-800 we can still derate, but we don't use assumed derate, just derate...for contamination, we use max thrust 26K in my company's case
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ThirtyEcho
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 8:47 am

"Wrong. Don't make me point you to the conveyor belt thread. Wink"

Righto. When the pilot steps on the right pedal to give the engines gas and uses the left pedal to shift into high gear, the airplane can start sliding on a wet runway. It might take some fancy action with the steering wheel to keep the airplane pointed down the runway.

Is this 2007 or 1927???

Watch out for those "air pockets," folks.
 
PhilSquares
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 12:33 pm

Quoting Qantas744ER (Reply 16):
Well high amounts of water going into the engine can lead to a surge and for me that can affect climb rate so on many aircraft CON ignition is turned on such as on the 744 so that the ignition will immediatly re-ignite the engine. Very good feature and many airline SOP's such as CX require this to be turned on if their 744 and all toher planes that have the feature when its raining hard.

On the 744 continous ignition is activated when the ignition switch is in Auto (normal position) and
1) TE flaps out of the up position
2) Engine Anti-ice on
3) N2 less than 50% (PW)
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bond007
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 1:05 pm

Quoting Pilotboi (Reply 13):
He's not necessarily wrong. It's not a wet surface that will cause a longer takeoff, but water in general, or any contamination (snow, dirt, debris, etc.). It causes a slower acceleration which in turn causes a longe takeoff roll.

Probably being dumb here, but why would water cause slower acceleration?

Snow, dirt,debris I can understand, but just water, even pooled, is not affecting how the wheels perform is it? If anything, it's possible it would cause faster acceleration if the friction caused by wheel to runway is less than wheel to bearing (therefore wheel skids a little).

The only thing that might occur is that the the surface tension of the water is 'pulling' on the wheel and slowing it's ability to turn efficiently ... is that it?

Jimbo
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flight152
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 2:51 pm

Quoting Pilotboi (Reply 13):
But of course this is a plus for landing!

Not at all. The lower coefficient of friction between the tires and runway on a contaminated surface would only reduce braking effectiveness.
 
AirportSeven
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 3:13 pm

From AC 150/5200-30B

"Snow, slush, ice, and standing water impede aircraft acceleration by absorbing energy in compaction and displacement. For aircraft decelerating, slush-covered pavements and, especially iced surfaces, hamper deceleration rates due to a reduction in the friction coefficient of the runway and the potential for hydroplaning. Standing water can also limit operations, although acceptable limits vary by aircraft, since most jet aircraft flight manuals limit the aircraft to landing on no more than ½ to 1 inch of standing water."
 
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Starlionblue
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 3:53 pm

Quoting Bond007 (Reply 18):
Snow, dirt,debris I can understand, but just water, even pooled, is not affecting how the wheels perform is it? If anything, it's possible it would cause faster acceleration if the friction caused by wheel to runway is less than wheel to bearing (therefore wheel skids a little).

There may be some friction yes. But the wheels do spin and not just skid over the surface. The effect is presumably negligeble.

For deceleration, of course, it's a different matter since wheel brakes are being used.
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BAE146QT
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 5:54 pm

[quote=Bond007]just water, even pooled, is not affecting how the wheels perform is it? If anything, it's possible it would cause faster acceleration if the friction caused by wheel to runway is less than wheel to bearing (therefore wheel skids a little).quote]

The roads around here have a significant camber, and bad drainage. As a consequence, the center of the road will be wet, but the shoulders of the road will have standing water/puddles when it rains.

Hitting these puddles in one of my cars, (it's very light) will cause a serious deviation toward the shoulder if I am not prepared for it. The drag that even a small amount of water can cause is quite impressive because, as AirportSeven quotes, "[...]standing water impede[s] aircraft acceleration by absorbing energy in compaction and displacement. "

I realise that planes and cars deviate in terms of tyre tread pattern, and later in the takeoff roll an aircraft will have less weight on the wheels, but I would still say that water is significant.
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bond007
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RE: Take Offs In The Rain?

Fri Oct 12, 2007 8:49 pm

Quoting BAe146QT (Reply 22):
Hitting these puddles in one of my cars, (it's very light) will cause a serious deviation toward the shoulder if I am not prepared for it. The drag that even a small amount of water can cause is quite impressive because, as AirportSeven quotes, "[...]standing water impede[s] aircraft acceleration by absorbing energy in compaction and displacement. "

Yes, I guess I thinking smaller pooling of water ... and yes, even my huge boat of a car has the same effect when hitting a pool of water, but we're talking big puddles here, not what I was thinking of having on grooved runways...

...but I'll stand corrected  Smile

..I still say it's negligible unless it's coming down at 10 inches/hr.


Jimbo
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HAWK21M
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RE: Take Offs In The Rain?

Sat Oct 13, 2007 10:03 am

Quoting PhilSquares (Reply 15):
there are some MEL items that can dictate that, for instance anti-skid inop

Whats the MEL on INOP A/S system out there during rains.Out here A/S is no go if Rain conditions exist at Departing & Arriving stations.
regds
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BAE146QT
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RE: Take Offs In The Rain?

Sun Oct 14, 2007 11:16 pm

Quoting Bond007 (Reply 25):
...but I'll stand corrected

Sir, I am no professional pilot by any means. I also appreciate that a concrete runway is usually a better maintained surface than a Northamptonshire road.

However, water does have a tendancy to pool if given the chance and, as is evidenced by photos here in the database, runways can get very wet.

Water doesn't like being compressed and it will resist being shifted. On takeoff, energy in moving water is energy unavailable for accelerating the aircraft.

As to what measure of water is required for it to be a factor, I don't know. I'll shrug and say that the FAA seem to think it's worth bearing in mind. I am interested in knowing why this is though;

Quoting PhilSquares (Reply 15):
In fact, on a contaminated runway, there are some aircraft that actually reduce thrust to increase performance with respect to VMCG.
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Ward86IND
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RE: Take Offs In The Rain?

Mon Oct 15, 2007 12:26 am

What aircraft do you usually fly on? It's possible you are used to AS's 737NG's which use up less runway and climb much quicker than the 734.
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CosmicCruiser
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RE: Take Offs In The Rain?

Mon Oct 15, 2007 1:20 pm

Quoting Geo772 (Reply 4):
On shorter runways in the rain (compared to dry) the takeoff roll is likely to be quicker because you have to allow for more runway to stop on in the event of a rejected takeoff

The T/O roll isn't shorter you just lower V1. Vr & V2 stay the same. I've never flown any jet that had diff climb parameters in rain. In the MD-11 the ign is always on automatically for the t/o anyway. I've never in my experience senn any jet engine surge because of water injestion, not to say it won't happen. I did have both eng surge in a Beech-18 once in a heavy rain and it definitely got my attention.
 
Ward86IND
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RE: Take Offs In The Rain?

Mon Oct 15, 2007 4:15 pm

Quoting CosmicCruiser (Reply 29):
The T/O roll isn't shorter you just lower V1. Vr & V2 stay the same.

I think what he meant is that the takeoff roll is shorter due to using normal thrust as opposed to reduced/FLEX takeoff power in the event of standing water.
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