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Tony747-400

Climb Gradient %

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Hi Guys

 

On a SID Chart I have an instruction stating- Minimum Climb Gradient 4% to 3000 Feet.

 

My question is, what figure is the 4% based on, and once I have the climb gradient figure in Feet per miniute how do I enter it into the FMC, or would I climb out using V/S Mode?

 

Thanks

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Thanks.

 

So on an SID chart, would you use the V2 speed to calculate the % into FPM? Also I guess you would have to climb in V/S mode as to my knowledge that can't be inserted into the FMC and use VNAV?

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Climb rate for a given minimum climb gradient is calculated using your GS.

 

For example, you want to get the desired Climb Gradient of 4% and you're having a 150kt GS, then --> 150*4 = 600fpm. That's your minimum climb rate for your given GS, in order to comply with the SID.

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Thanks.

 

So on an SID chart, would you use the V2 speed to calculate the % into FPM? Also I guess you would have to climb in V/S mode as to my knowledge that can't be inserted into the FMC and use VNAV?

 

SID procedures only ask for a minimum climb gradient, which in you case would be 4%. It means you can to climb at or above that 4% gradient. There's no need to use V/S mode. Just calculate the gradient and if the airplane can comply with it, just fly the normal climb procedure of the airplane you're flying and you'll be fine. If the gradient is above the airplane's capability, you must communicate this fact to ATC so that they can adjust the procedure to your situation or assign you a different SID. Most airliners can cope with high climb gradient restrictions, so you'll be good to go on most of the procedures around the world.

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Ok

I've got the part now of how to work out the % Gradient FPM, I will use the V2 speed.

Can someone tell me though in the real world would the climb be made using V/S Mode up to the required altitude then switch over to VNAV. I can't see any other way of doing it unless someone can tell me different.

SID procedures only ask for a minimum climb gradient, which in you case would be 4%. It means you can to climb at or above that 4% gradient. There's no need to use V/S mode. Just calculate the gradient and if the airplane can comply with it, just fly the normal climb procedure of the airplane you're flying and you'll be fine. If the gradient is above the airplane's capability, you must communicate this fact to ATC so that they can adjust the procedure to your situation or assign you a different SID. Most airliners can cope with high climb gradient restrictions, so you'll be good to go on most of the procedures around the world.

 

Thanks very much, understand it now

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SID procedures only ask for a minimum climb gradient, which in you case would be 4%. It means you can to climb at or above that 4% gradient. There's no need to use V/S mode. Just calculate the gradient and if the airplane can comply with it, just fly the normal climb procedure of the airplane you're flying and you'll be fine. If the gradient is above the airplane's capability, you must communicate this fact to ATC so that they can adjust the procedure to your situation or assign you a different SID. Most airliners can cope with high climb gradient restrictions, so you'll be good to go on most of the procedures around the world.

Assuming all your engines are working ;)

 

When you're in a Turboprop and the critical engine fails... then it gets interesting :P

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Thanks for that because up until now I've always ignored climb restrictions on charts as I haven't known how to calculate climb percentage. Usually if you follow the flight directors that puts you above minimum climb rate anyway.

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Isn't it a bit to easy to say IAS=GS? I guess you can get away with IAS=TAS close to the ground, but nonetheless you should calculate GS as TAS +/- wind.

 

(If you where at IAS 140 kts at 12000 ft. your TAS would be 140*1.24 = 173,6 kts.)

 

Kyle? ;)

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http://www.pprune.org/tech-log/449863-tas-ias-pa-oat.html#post6419000

Formula for altitude and temp correction of TAS.

 

There is also a table in FCOM vol1 for one engine out go-around climb gradient with all corrections. According to it basically you'll lose about 0.9% per every 2000'. And there is probably an engine out procedure chart with other or even without minimum climb gradient.

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So the reasons we have minimum climb gradients are several, but some of the main - and more obvious ones - are terrain and anticipated traffic flows underneath the departure. Instead of climb gradients, for the latter, you'll also see at or above altitudes (altitudes with a bar below them: 2000).

When it comes to many of these (but not all), they're easily accomplished by modern jets and props. The ones more at risk are your smaller GA aircraft, particularly on hot days. Those are the ones who really need to do the calcs.

A 4% climb gradient (rise/run, or feet gained per nm; it's another way to say 2.3 degree climb angle) is 243.0447 ft/nm. At a ground speed of 180 knots, that's 729.1342 feet/min. As long as you can maintain a vertical speed of 730 or greater, you're good.

How that is accomplished is up to you. In order to ensure compliance, I would use V/S until the altitude specified, and then cut over to VNAV once the restriction was cleared. I wouldn't necessarily use VS to set the minimum climb gradient (remember, it's only a minimum). I would use VS to ensure VNAV doesn't dip below the value (it probably wouldn't, but I'd rather know for sure), and set the VS in a similar pitch-for-speed fashion.

Insert math here:

1 nm = 1.15078 mi
1 mi = 5280 ft
1 nm = 6076.118 ft

Assuming 180 knots groundspeed and 4% climb gradient:

(6074.118) * 4% = 243.0447 ft/nm

180 knots (per hour) / 60 min = 3 nm/min

243.0447 ft/nm * 3 nm/min = 729.1342 ft/min

'*Yes, I forgot everything about significant figures after HS Chem class.
'No, I really don't care

Isn't it a bit to easy to say IAS=GS? I guess you can get away with IAS=TAS close to the ground, but nonetheless you should calculate GS as TAS +/- wind.

(If you where at IAS 140 kts at 12000 ft. your TAS would be 140*1.24 = 173,6 kts.)

Kyle? ;)


Haha - me?

And it depends on how you looks at it. Sure, IAS, TAS, and GS will be relatively close while at lower altitudes, wind does play a factor, as you mentioned. The issue, though, is precision.

With a 180 knot GS and a 4% min climb gradient, you're looking at a min of 729 feet/min. Add in a 5 knot headwind and you're looking at a marginally better 709. With a 10 knot headwind, it jumps further down to 688. While those are rather different numbers, consider how you're going to follow that.

You could, of course, fly this yourself, but the VSI isn't really going to give you the precision you're looking for to find 730, or even 750 with precision. Since this is the general forum, I'm assuming all PMDG fleet members here, and the only one I can think of that utilizes 50' splits is the MD-11 (up to 1000' of VS). So, in order to hit the min climb gradient, for 0 and 5 knots of headwind, I'm going to use the 750 VS setting at a minimum in the MD-11, and 800 in the Boeings/J41. This is, of course, only if you wanted to come in right at the bare minimum:

office-space-flair.png


...but in the end, I'd 100% agree with you that you should add in all the factors so that you knew the minimum climb gradient for the absolute worst circumstance. It's worth knowing, even if you're - hopefully - not going to sit right on the number itself.

Then again, with some aircraft on a hot day in Aspen...

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When you're in a Turboprop and the critical engine fails... then it gets interesting :P

I highlighted the word "critical" in that statement, as it is more important than the mere fact you lost an engine. Not all engines are equal (and it is related to direction of rotation of the propellor, and whether it is counter-rotating to the opposite side).

 

Aircraft where the props rotate the same direction have a critical engine. Aircraft that have props rotating in opposite directions may have two or no critical engines (depending how you count).

 

The definition of critical engine is thus:

 

"The critical engine is the engine which, following a failure, will result in the greatest adverse effects from the remaining engine".

 

If you look at the rear of the aircraft, facing forwards, if both propellers rotate the same direction (let's say clockwise) then the critical engine is on the LEFT (the right-hand engine will produce the greatest turning moment as the thrust is located furthest away from the CoG).

 

This matters not only because of controlability, but drag as a result of larger control deflections.

 

All you need to know is the climb gradient as per the operating manual for single-engine climb with critical engine INOP (twin-engine aircraft) or dual-engine climb (quad-engine with two engines out on the same side). If you can climb in this configuration and still meet the minimum climb gradient quoted for the departure, then you can depart, otherwise you need to find another way out of where you are.

 

As for the actual gradient - the charts assume zero wind conditions. You must allow for wind in your performance calculations.

 

I see Kyle beat me to the math. :)

 

Best regards,

Robin.

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Climbing out in VS mode is a ridiculous waste of aircraft performance, which will keep you unnecessarily close to the terrain you are supposed to be avoiding. It also puts the autothrottle in speed mode when it should be in thrust.

 

You should calculate the minimum rate of climb and monitor vertical speed to ensure you exceed it. If necessary I would use FLCH rather than VNAV to ensure I had full control of airspeed which gives you full control of vertical speed.

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ROC/ROD = GS X %

 

Ex : rate of descent for a grounspeed of 120 kts and 5% glide (=3°)

 

ROD = 120 X 5 = 600 FPM

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