3 to 1 Rule and PDP ?

[your_dreamguy]

Hello,

I have two questions on these "rules of thumb."

First, can someone clearly explain the 3 to 1 rule. I looked in the "Rules of Thumb" section and the example they gave looks different than the this example:

You need to descend from FL330 at 300 KIAS to 17000 at 250 KIAS. What distance will it take?

33000-17000=16000
16000*3=48000
48000/1000=48NM
48NM+5NM = 53NM

I just copied this example because I didn't understand it. I understand how they got 48NM but where did they get 5 NM? Is 5, 10% of (300-250)?

Second,
Can anyone tell me what the acroynm "PDP" stands for and give an emample it's use?

Any help is appreciated. Thanks

 

[EOpilot]

The 3 to 1 rule is just a rule of thumb for a Planned Descent Profile. If you need to cross a certain point at a certain altitude you use this rule to help plan when to begin the descent.

It takes about 3 miles to descent 1000 feet at a rate of descent of about 5 x your groundspeed (or half your groundspeed times 10).

The 5 miles added on was for the speed reduction. Your formula is correct, but an easier way to put it is about one mile for every ten knot reduction in speed.

 

[Super 80]

In the 727, the three to one rule works pretty good for idle descent from cruise Mach to 320-330 kts, (although when we had them we used barber pole descents if the air was smooth and we made gas).

On the Super 80 it works much the same except the ice machine will do better than three to one from the thirties to the mid twenties when you switch over to IAS (suggested at 310).

The five miles was the round out factor, as a constant it adds in the push over and pull out to provide a margin of comfort and assurance of making your restriction.

Speed reductions are usually added in on top, and at idle, 1nm per 10 knots works. Using the low limit function (1.10 EPR) on the not-so-auto throttles on the 80 uses about 7 miles for a 40 knot reduction (250-210 on the DFW arrivals).

Also figure in head/tail wind. One gouge that works is -1/+1 for every 10 knots of wind respectively.

Hope that helps.

P.S. Four to one works pretty good with a constant 2000 vvi descent at cruise mach/310 kts. For an 10,000 foot descent subtract 1,000 = 9,000. Drop the zeroes and times four equals 36. Add in the cruise mach you'll be holding at the L/O as miles i.e. .8 = 8 miles, .5 = 5 miles for the last thousand feet at 1000' vvi. (36 + x = ) Put in a round out constant (+5) and compensate for winds (+/-) and you'll have a nice descent that'll make your passengers feel like they've never left their living room.

 

[Flying Illini]

simply put, the 3-1 rule is a ratio. If I'm at FL300 and the airport is at sea level then I have 30,000' to lose. How far out do I want to descend? Mulitply altitude to lose (30,000) by 3 to get 90,000. Since the thousands don't matter you can drop them. 30*3=90. I want to begin my descent no later than 90 miles out. In this example if I haven't been given lower by 100nm, I start asking for lower.

 

[Mr. Cole]

Here is a proof I put together for the 3-1 Rule and the 3 degree glide slope rule. Hope it helps.

Dave

Proof for 3 Degree Glide Slope Rule of Thumb

Imagine a triangle where the hypotenuse represents your flight path, and the desired angle of descent is 3 degrees. Your movement vertically (altitude) is related to your movement horizontally (across the ground) by a fixed relationship. Namely,

Tangent (3 degrees) = Altitude/Horizontal Position,

where the Horizontal Position represents your current distance from the point of desired touchdown and the altitude is the height above that point. However, this same relationship must also apply to the rate of change of these quantities over a fixed period of time. So if altitude is changing by a fixed amount over time, let’s say per minute, then the motion across the ground has to be such that the above relation holds. Another way to write this is:

Tangent (3 degrees) = Descent Rate (fpm)/Ground Speed (fpm)

This can be rearranged as Descent Rate (fpm) = Tan (3 degrees) * Ground Speed (fpm)

So let’s convert the ground speed in knots into fpm. The first step is to convert knots into feet. Let’s approximate 1 nm as 6000’. So Ground Speed in feet/hour is 6000* Ground Speed (knots). Now we want to make the unit of time consistent with the descent rate, which is in minutes. This can be done by dividing the Ground Speed (feet/hour) by 60. The result is Ground Speed (fpm) I= 100*Ground Speed (knots).

Plugging this back into the equation for Descent Rate (fpm) gives:

Descent Rate (fpm) = Tan (3 degree) * 100* Ground Speed (knots).

Tan (3 degrees) is approximately 0.05, so plugging this in the above equation yields:

Descent Rate (fpm) = 0.05 * 100 * Ground Speed (knots) = 5* Ground Speed (knots)


Proof for 3 Degree Glide Slope Rule of Thumb

So we want to descent from a particular altitude, say XY,000 feet. We could also write the starting altitude in terms of thousands of feet as XY(thousands of feet) *1000. We know what the distance is we want to descend through vertically, and if we assume a 3-degree descent based on the above relation, the time it will take to descend that distance is:

Time (min) = Altitude to Descend/Descent Rate (fpm) = XY *1000/5*Ground Speed (knots).

In that time you will travel a distance across the ground, Distance (Across Ground) = Time (hours) * GS (knots).

We can compute the time in hours by dividing the formula for Time (min) above by 60.

So Time (Hours) = XY*1000/60*5*Ground Speed (knots). If we multiply Time (Hours) by Ground Speed, we see that the Ground Speed cancels out.

The Distance Across the Ground = XY*1000/60*5 = 3.33 * XY, where XY is altitude in 1000’s of feet.

 

[WrightAvia]

Who needs a 3 into 1 rule...slam dunk!

At 0149:18, the CVR recorded the sound of an electric seat motor. FDR data indicated that, at 0149:45 (27 seconds later), the autopilot was disconnected.12 Aside from the very slight movement of both elevators (the left elevator moved from about a 0.7° to about a 0.5° nose-up deflection, and the right elevator moved from about a 0.35° nose-up to about a 0.3° nose-down deflection)13 and the airplane's corresponding slight nose-down pitch change, which were recorded within the first second after autopilot disconnect, and a very slow (0.5° per second) left roll rate, the airplane remained essentially in level flight about FL 330 for about 8 seconds after the autopilot was disconnected. At 0149:48, the relief first officer again stated quietly, "I rely on God." At 0149:53, the throttle levers were moved from their cruise power setting to idle, and, at 0149:54, the FDR recorded an abrupt nose-down elevator movement and a very slight movement of the inboard ailerons. Subsequently, the airplane began to rapidly pitch nose down and descend.

Between 0149:57 and 0150:05, the relief first officer quietly repeated, "I rely on God," seven additional times.14 During this time, as a result of the nose-down elevator movement, the airplane's load factor15 decreased from about 1 to about 0.2 G.16 Between 0150:04 and 0150:05 (about 10 to 11 seconds after the initial nose-down movement of the elevators), the FDR recorded additional, slightly larger inboard aileron movements, and the elevators started moving further in the nose-down direction. Immediately after the FDR recorded the increased nose-down elevator movement, the CVR recorded the sounds of the captain asking loudly (beginning at 0150:06), "What's happening? What's happening?," as he returned to the cockpit.

The airplane's load factor decreased further as a result of the increased nose-down elevator deflection, reaching negative G loads (about -0.2 G) between 0150:06 and 0150:07. During this time (and while the captain was still speaking [at 0150:07]), the relief first officer stated for the tenth time, "I rely on God." Additionally, the CVR transcript indicated that beginning at 0150:07, the CVR recorded the "sound of numerous thumps and clinks," which continued for about 15 seconds.

According to the CVR and FDR data, at 0150:08, as the airplane exceeded its maximum operating airspeed (0.86 Mach), a master warning alarm began to sound. (The warning continued until the FDR and CVR stopped recording at 0150:36.64 and 0150:38.47, respectively.)17 Also at 0150:08, the relief first officer stated quietly for the eleventh and final time, "I rely on God!"

 

[GeorgeTG]

PDP

PDP = Planned descent point

The PDP is used for flying Constant Angle Non Precision Approaches (CANPA) and marks the "top of descent".

Let's say you cross the VOR at 3100 ft
The MDA is 1000 and the TDZE is 550

Let's say the MAP is at 10DME from the VOR

You have 2100 ft to loose to the MDA, and an additional 450 ft to touchdown for a total of 2550 ft.

At a constant 3 deg descent you will need approximately 2550 / 300 = 8.5 NM.

If you begin your descent at the VOR, you will have to level off at the MDA and fly level for 1.5 NM before continuing your descent.

Instead, you maintain 3100 ft crossing the VOR and begin your descent at 1.5DME after the VOR, this is called the PDP or Planned Descent Point.

I've oversimplified it here, but you can imagine how a CANPA smoothes out those approaches with 5,6,7 stepdown fixes.

If you work out a PDP you'll also get the VDP as part of the deal...

Cheers
George

 

[Iceman21]

CANPA approaches are not approved unless you have a L-NAV equipped airplane.

I discussed this at length with my DE during my CFI-I checkride. He swore by them and they do work, however, there could be a situation where you would have to descend below MDA using CANPA for you to arrive at the MDA at the MAP, especially in the situation with 5 or 6 step-down fixes.

 

[Timebuilder]

That's a neat proof. Once you use the short rule-of-thumb version a few times, you'll see how well it works for descent planning.

I recognize the above post from the EgyptAir crash investigation.

...the relief first officer stated quietly for the eleventh and final time, "I rely on God!"

Just a note. When you place your faith somewhere, whether it be a short formula for descent or a deity, think long and search your heart about WHERE you are placing that faith, just as you would do a good preflight inspection to determine if you could reasonably expect a safe and noneventful flight from your aircraft.

 

[metrodriver]

3:1 thousand feet per 3 miles or 300 feet per mile.
So to plan the descent: altitude to lose times 3 plus one mile for every ten knots of airpeed to slow. Vertical speed will be groundspeed divided by 2 plus a zero (400kts will give you 2000fpm).
This same 300'/ NM plays part in calculating a VDP for a non-precision approach under 121/135 where you are required to make the final approach to landing using normal procedures (no slam dunk). So if your MDA is 400' above TDZE you need to see the runway at 1.3 miles from the threshold (unless the final approach has a non-standard approach path - more than 3 degrees).If you have to time subtract 10% of the time required for the approach to come up with this VDP.
If there is a published VDP you have to wait till you reach that, it takes priority over your own calculated one.

 

[TurboS7]

When my FMC says TD.

 

[Mr. Cole]

Timebuilder,

I did it one day while bored at work.

Dave