Non Precision Approach Page 2
At what altitude should we join the arc?
Well what we could do is just descend and intercept the arc at the enroute MSA or DME step and then descend in accordance with the published arc min altitudes. However, would this be the most efficient way to descend? Well Yes and No. It really depends on what the weather is doing. If there is a chance of going visual then you will want to be down as soon as possible in order to sight the field and then continue to land visually. If however the weather was say BKN007 at 6000m then a full approach will be required and we can plan our approach accordingly. If we descend too early we will use more fuel and besides it is not nice for the punters in the back as the engines would be spooling up and down and the cabin angle pitching all over the place as we dropped down the descent steps and leveled out until reaching the next step down.
Ideally we need to calculate a descent point that will allow us to descend at a near constant rate throughout the descent and arrival phase.
Learn your 3 times table.
Most high performance pressurised aircraft need approximately 3 nautical miles (nm) distance per 1000 ft of altitude to lose (remember to deduct the height of the airfield from the cruise altitude as not all airfields are at sea level. In Napier’s case we can assume it to be sea level as it is so close).
Lets assume we are cruising at FL160 (16000 ft).
We need to lose 16000 ft @ 3 nm per 1000ft
16 x 3 = 48 nm
If cruising at FL200:
20 x 3 = 60 nm
For non-pressurised aircraft divide your GS by 60 and multiply by 2 and this will be your distance per 1000 ft to lose.
Example
A piston twin descending at 180 kts. Nil wind so airspeed approx equals groundspeed
Divide the GS by 60. This will gives us the number of miles we travel per minute.180/60 =3
This means the aircraft will travel 3 miles for every minute. For non-pressurised aircraft the rate of descent should be around 500ft per minute to avoid ear and sinus discomfort.
In our case we are traveling at 3 nm per minute and during that minute we will have descended 500 ft. After 2 min we will have descended 1000ft alt and traveled 6 nm.
Therefore dividing the cruise alt by 1000 and multiplying by 6 will give the ideal distance that the descent should be commenced. You can of course also use a TOD (top of descent point) based on time but this will be trickier to monitor for an arc approach.
Ok so now we have worked out our TOD point and for the purposes of this exercise we will use FL200 with a calculated descent distance of 60 nm. We could start our descent at 60 DME NR but would this be the most efficient descent point? No. The reason being is that we have 2 other things to consider:
- The distance to travel around the arc.
- The DME steps on the route and altitudes restrictions on the arc.