Why do holds at or below 14,000 feet have different procedures?

When holding at or below 14,000 feet, the time for each leg is 1 minute. However, above that altitude, the legs are flown for 2 minutes, and the turns for 1 minute and half. Are these differences due to airspeed, or any other recommendation?

The reason for a different turn rate at higher altitude is because of the physics of Indicated (IAS) and True (TAS) airspeed. Many aircraft would find it impossible to do a standard rate turn at high altitudes.

A standard rate turn is 3° per second. This is also known as a two-minute turn because it would take 2 minutes to make a full 360, or 1 minute for a 180°, like each end of a holding pattern.

The bank angle required to make a 3°/sec turn is a function of TAS and bank angle. The formula is:

$bank\,angle =\frac{turn\,rate\cdot TAS}{1091}$

So a standard rate turn at 170kn TAS requires a bank angle of 25°. As bank angle increases so do load factor and stall speed. A 25° bank yields a 10% increase in load factor and a 5% increase in stall speed. For this reason FAA and ICAO regulations require holding patterns to be standard rate, but not to exceed a 25° bank angle.

Here's where the altitude enters the equation. The relationship between TAS and IAS is dependent on air pressure and density. At 5000 feet in standard atmosphere an IAS of 160kn yields a TAS of 170kn. A standard turn at 170 KTAS would require the maximum bank angle of 25°. But in the thinner air at 14,000 that same 25° bank at 170 KTAS is only 142 KIAS.

So as altitude increases, a standard rate turn requires a slower IAS and a higher bank angle. At higher altitudes it would become impossible for many aircraft to make a standard rate turn without stalling. So at 14,000 feet the required turn rate is decreased to 2°/sec, which is a three minute turn (thus 90 seconds for a 180°). Where a standard turn at 14,000 IAS is limited to 142 KIAS, a 3 min turn can be performed up to 212 KIAS.

This is the reason turn rate must be lowered at higher altitudes.

I gave up on trying to figure out why the designers do what they do. However, here’s a guess on this one.

If you are holding below 6,000' you are most likely on an approach to an airport or holding at the missed approach point. By limiting speed to 200kts and legs to 1 minute the protected area is much smaller. Often you will also see a note on the chart limiting the distance from the holding fix. The designer can make the hold closer to the airport elevation which makes the approach easier for the pilot. The purpose of the hold on an approach chart is for a course reversal or missed approach. On other forums, general aviation pilots have stated that they have never, or almost never, been given a hold for spacing when on an approach.

If you are holding above 6,000' you are most likely not a Cessna 152—you are probably in a much faster airplane. So the protected space needs to be substantially larger. That’s why the airspeed is limited to 230 kts between 6,001' - 14,000’ and legs are 1 minute 30 Seconds.

Above 14,000' you are probably an airliner or business plane that is holding for spacing purposes. The highest mountain peak in the continental US is 14,505' and we know that in mountainous areas the MEA is 2,000' above the highest obstacle. So a hold along an airway has 4 miles on the holding side already protected. And outside of California and Colorado there is plenty of room. ATC can have an aircraft hold without worrying about them hitting the ground. But limiting airspeed to 230 kts and making legs 1 minute 30 Seconds makes it easier to ensure that the airplane won’t hit anyone else.

Holds used to be much more common in the US airspace but the FAA computers do a much better job of estimating traffic and the implementation of ground stops and EDCTs has made them much less common than in the past. From what I have read on other blogs, they are mostly used for unanticipated weather delays.