Don’t be descending blindly in VMC conditions

No sh*t the FMS finally stopped working!

I’ll beat you “Children of the magenta line” commentator hopefuls to the punch, it was just a whatever moment even though I am not the oldest dude.

With an In-Op FMS the only change to our flying was that we would be on the jet airways in the VOR mode until we began our descent. Oh, and we had to let ATC know that we were /A only with only DME and the VOR’s

But when were we going to start down and at how many feet per minute?

With an In-Op FMS the only change to our flying was that we would be on the jet airways in VOR mode until we began our descent. Oh, and we had to let ATC know that we were /A only with only DME and the VOR.

Luckily, we still had an operational DME so finding our ground speed wasn’t difficult, with the rule of 36 seconds to calculate our speed….

It’s simple! Start a timer and see how far you travel in 36 seconds then multiply that number by 100… So let’s say you went 3.0 miles in 36 seconds… So you’re flying at a groundspeed of 300 knots.

For example let’s keep it simple with these numbers:

• 300 Knots GS
• 33,000 FT Cruising Altitude
• Airport Elevation of 1,000 Feet.

Now let’s calculate when we need to descend! Figure out how much altitude you have to lose, in our example, we would subtract our airport elevation from our cruising altitude. If you are fairly smart you should end up with 32,000 feet. Now drop the zero’s and multiply the number 32 by 3 and you would be at 96. So 96 miles from the airport we will be at our top of descent.

Last step, how many Feet Per Minute do we descend?

This is also fairly simple. Take your groundspeed which is 300 knots divide it by 2 and add a zero! You should get 1,500 so will initially be descending at 1,500 FPM. If we slow down or accelerate you have to use the groundspeed calculation again. This descent rate will change for the entire descent if we want to be on the correct altitude at our bottom of descent.

So 96 miles from the airport we will begin a 1,500 fpm descent!

How many feet per minute if we were doing 400 knots? 2,000 FPM

How many feet per minute if we were doing 200 knots? 1,000 FPM

How many feet per minute if we were doing 600 knots? 3,000 FPM

The descent example we used is actually a 3.0 degree descent angle. You can use the formula for ATC crossing restrictions, calculating descent rates for non-precision approaches, and for knowing when to begin your descent!

This calculation would change slightly for faster airplanes like jets. You would need to plan to slow at 10,000 feet to 250 knots so you would add a 5–10 miles to the 96 miles to slow down.