3 Degree Glide Path concept? Why 700 fpm Rate of Descend?

[z.khalid]

So I have been lurking around trying to find the perfect answer for this question.
A 3 Degree glide path is basically losing 300 feet for every nautical mile when you're at a speed of 60 kts.
For 120 kts you would be losing 600 fpm to maintain that 3 degree glide path.
For 130-140 it is more or less 700 FPM rate of descend which is used generally on the 737 on approach.
Do I have the concept right?

Please do correct me, explain further.
I just want to know for my own knowledge.

Thanks all.

[Lord Spandex Masher]

You will lose the same amount of height per unit of distance on a constant three degree slope regardless of ground speed.

At higher speeds you will cover that distance in a shorter time so you're rate of descent will have to increase to maintain the three degrees.

Approach speeds of airliners also vary between about 100kts and 160kts (ish).

As a rule of thumb if you multiply your groundspeed by five you will get your required rate of descent for a three degree descent.

[FlyingStone]

Math explanation:

3° = 5,24 %

1 NM = 6076ft
6076 x 0,0524 = 318,4 ft/NM

Basically, as LSM says, if you go faster, you have to descend faster as well. The rule of thumb is ROC = GRAD x GS, which means for 3° glide slope, you multiply your groundspeed in knots by 5 and you get the rate of descent in ft/min.

[Globally Challenged]

Just for the sake of ease of mental arithmetic - I prefer the 2 times table. So divide your GS by 2 and then add a 0 - you get the same result

[z.khalid]

Makes a lot more sense now, thanks a lot for the explanation guys.

I am not Type Rated, so I don't have a lot of knowledge on how things work on a jet aircraft, but I am just doing a lot of reading.

On a Precision Approach (ILS) I would assume it's easy to descend down and let the auto pilot or you manually follow the GS.
On a NON precision however(A VOR or LOC approach), how do you calculate your Flight Path Angle?

Finally(a different question), when flying a VOR approach for example, with no DME, how do you know how long you would time for?
I am very confused on this kind of approaches.

Thanks in advance guys.

[Otto Throttle]

For non-precision approaches, the relevant information is contained on the briefing plate.

The equivalent glide path or flight path angle is usually given in both degrees and % next to a table indicating required rate of descent against ground speed. Where an approach is based on dme distances, there are also various check altitudes against distance so that the accuracy of the approach can be judged at regular intervals and corrective action taken where required.

Where no dme is associated with the approach, this table of RoD and GS should also contain the equivalent timings from a specified point on the approach. This fix would usually be some kind of marker beacon where no dme is used.

[hvogt]

In both cases, precision approach and non-precision approach, you would estimate the required rate of descent using the formula ROD = GS * 5 or (more accurately) ROD = (GS * 5) + 10%.

In a precision approach your vertical guidance, indeed, comes from the glideslope. However, when intercepting the glide slope you must have an idea of your required rate of descend. Otherwise you will just end up chasing the horizontal needle.

In a non-precision approach you would normally establish the required rate of descend, and then check your altitudes at intervals of normally 1 NM. If you start your final descend at, let's say, 3000 ft altitude and 9 NM from touchdown then you would check at 8 NM from touchdown if you are on your profile, i.e. at 3000 ft - 300 ft = 2700 ft. Normally approach charts have tables for these checks.

For NPAs without DME you would use a timing table on the approach chart.

[ReverseFlight]

It doesn't matter if you've never flown a jet - the 2 times rule and the stuff mentioned by hgovt works equally with your GA aircraft. I think you might need to revise some of your basic instrument theory.

[FlyingStone]

Finally(a different question), when flying a VOR approach for example, with no DME, how do you know how long you would time for?
I am very confused on this kind of approaches.

I seriously hope you don't have the IR rating yet, since this is realy BASIC knowledge and you couldn't possibly make it without this.

Time to VDP = (Altitude at FAF - MDA) / Rate of Descent
Time to MAP = Distance from FAF to MAPt / ( Groundspeed [kt] / 60)

Note that in this case you should use last available fix for timing (such as outer marker or NDB/locator in final approach) if it's not located with FAF, to get the shortest times as possible, since this is much more accurate.