1. TCC
Junior Member
Join Date
Mar 2005
Location
Taiwan
Posts
83

Hi,

Here is an example:
Southampton SID of London City Airport:
RWY 28: Maintain minimum climb gradient of 7.4% to 1100' QNH(1083' QFE)

In this case, how to execute "7.4%"?

Regards,
TCC

2. Member
Join Date
Mar 2005
Location
Dublin, Ireland.
Posts
600

at a wild crazy guess, I would say that it's the percentage of a 90 degree climb.... Though patently wrong. Given that it's London City we're talking about a, high climb gradient will probably be required.

Any sensible theories folks?

John

Insert droll or witty comment here.

3. VIP Member
Join Date
Mar 2005
Location
New York.
Posts
4,439

On a U.S. DP the climb gradient is expressed as the number of feet per nautical mile you are expected to climb. You then have to do some math. Its basically a slope.

That means that you should be able to climb 7.4% as many feet as the number of feet you travel forward. So for every statute mile (5280 feet) you travel over the ground, you must climb at least 5280 x .074 = 390.72 feet, call it 400 feet. This may be for terrain clearance, to guarantee radio or nav aid reception, to clear restricted airspace, or other reasons.

So how long does it take to cover a statute mile? At a 70 mph climb speed (ground speed, of course), it would be about 51.4 seconds to go a mile, so you'd need 390 feet in 51 seconds. 51 seconds is .85 minutes, so 390 / .85 = 458.8, so you'd need to maintain about 460-500 feet per minute climb rate at 70 mph ground speed. If your ground speed is 70 knots, then 1.15 x 458.8 = 527.62, so you'd have to be a little over 500 fpm in your climb.

Some places I've seen this expressed as feet per mile, rather than a percentage, which makes the math a little easier.

[HR]
Larry N.

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5. Member
Join Date
Mar 2005
Location
Oklahoma
Posts
232

Just to piggyback on what lnuss said, if your airspeed indicator is in knots, better use 6,000 feet instead of the 5280 as a NM is 6000 feet. Minimum climb gradients are used for obstacle clearance out of an airport. Multiply your Nautical Miles per Minute times your Feet Per Nautical Mile to determine the approximate climb rate you will need to have on your VSI. For example, if you will be climbing out at 120 kts, you are doing 2 NM/MIN. If the climb gradient requires a 7.4% climb gradient, 6000 * .074=444 FT/NM. Multiply 2 times 444 which equals 888. You will need to see at least 888 feet per minute on your VSI or you better have an idea what and where the obstacle is and have an alternate plan if you don't or won't have the performance.

Capt. Bob

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