# What does “1%” mean in ICAO Annex 14 § 3.9.9 (longitudinal slope changes)?

— 1 per cent per 30 m (minimum radius of curvature of 3 000 m) where the code letter is C, D, E or F; and
— 1 per cent per 25 m (minimum radius of curvature of 2 500 m) where the code letter is A or B.

Can someone explain what "1% per 30 m" means?

Does it mean 0.3 m of elevation difference for every 30 meters? Or 1/100 per 30 m?

• Percent means a hundredth – expeditedescent Jan 21 '20 at 5:55
• Thanks hougaard for your response!!! – Saima Hasan Jan 21 '20 at 9:23
• Assume if my slope is 0.75m in 50m ,will this criteria fits to 1% per 30m?? really appreciate if you can share your calculations for this? – Saima Hasan Jan 21 '20 at 9:25
• @SaimaHasan if your slope is 0.75m in 50 meters, that will be 0.75/50=0.015, which is 1.5% – Jpe61 Jan 21 '20 at 18:06

You are referring to ICAO Annex 14 Volume I, Aerodrome Design and Operations. Section 3.9.9 says:

Recommendation.— Where slope changes on a taxiway cannot be avoided, the transition from one slope to another slope should be accomplished by a curved surface with a rate of change not exceeding:

• 1 per cent per 30 m (minimum radius of curvature of 3 000 m) where the code letter is C, D, E or F; and

• 1 per cent per 25 m (minimum radius of curvature of 2 500 m) where the code letter is A or B.

To better understand this, lets see what the previous section says:

3.9.8 Longitudinal slopes

Recommendation.— The longitudinal slope of a taxiway should not exceed:

• 1.5 per cent where the code letter is C, D, E or F; and

• 3 per cent where the code letter is A or B.

So: for a taxiway with a code letter D for example, a maximum slope of 1,5% is recommended. That would be 1,5 meters up or down within a distance of 100 meters.

If a part of the taxiway in question is 1,5% uphill, and another part is 1,5% downhill, the transition from the uphill portion to the downhill part should not happen at a rate faster than 1% within a distance of 30 meters.

In addition to this, the radius of the curvature should not be less than 3000 meters. This means transitions must be smooth within that 30 meters distance.

The important word is "change" - that is, how much the slope is allowed to change - so no sudden sharp edges are allowed, but all connecting slopes (or flats) need to be rounded off by 3000 or 2500m. How much the allowed slope is - will be discussed elsewhere.