TOD Calculator for Sim Pilots

Calvin Waterbury
Posts: 199
Joined: Mon Nov 07, 2011 10:37 am

TOD Calculator for Sim Pilots

Post by Calvin Waterbury »

Hello,

I have been working on the following project for some two years. There are probably some who might have the answer in five minutes, but I am not a mathematician. ;) The gist is to come up with a "Rule of Thumb" (ROT) formula where after the speed, altitude, trip length and desired FPM descent are entered, the descent distance from TOD to BOD can be pin-pointed for any aircraft at any speed for sim pilots that don't know "when" to start the descent. I think it is generally accepted the flight profile will differ greatly between a Piper Archer and a SR-71, but if we look at proportions, is it possible the two might be close enough to identical to be useful in cases where no charts, graphs, tables, etc. are available, i.e., for sim pilots?

To date, I have been considering and toying with these things...
  • Flight Profile - What parts of the flight profile can offer the biggest fuel savings?
    • Takeoff
    • Departure Altitude or BOC (Bottom of Climb)
    • TOC (Top of Climb)
    • Cruise Altitude
    • TOD (Top of Descent)
    • Approach Altitude or BOD (Bottom of Descent)
    • Landing
  • What parts of the flight profile can offer the biggest fuel savings in descending order IMO? (The order is open for debate)
    • 1. Descent - By far the best fuel economy given this is essentially a glide, so maximizing this segment is desirable.
    • 2. Cruise - Especially for jets at high altitudes. Once at altitude, turbines "sip" fuel, but it takes a lot to get the birds to those altitudes.
    • 3. Landing - Mostly a low-powered glide, but this savings might be mitigated by reverse thrust and perhaps glide slope adjustments on final.
    • 4. Climb - This is an area where most of this discussion may gravitate. The real question is whether it is most efficient to climb faster to TOC, say thirty-five degrees to begin enjoying fuel savings at higher altitudes faster or to have a longer and less demanding climb, like fifteen to twenty degrees?
    • 5. Approach - Similar to Cruise, but at a lower, less-efficient altitude.
    • 6. Takeoff - The highest rate of fuel consumption at the least efficient altitude.
  • What criteria are we interested in?
    • Average Ground Speed
    • Altitude
    • Rate of Descent FPM
    • Total Distance
    • % Distance for Climb (Just kicking around an idea)
    • % Distance for Descent (Same as above)
    • "Stability Buffer" (My invention of added distance to allow APPROACH stabilization after the descent, if needed.)
    • What Rate of Climb (ROC) would a pilot want to use for best fuel economy and passenger comfort? -
    • What Rate of Descent (ROD) would a pilot want to use for best fuel economy and passenger comfort.
Generally, I am after a ROT formula which will yield the most economical flight profile.

Later on, I would like to massage a formula that could use True Air Speed (TAS) at TOD rather than an average Ground Speed which requires some guesswork on the pilot. The above is really overkill for what I am after, but I thought it might be helpful if I let on what I have been cogitating for some time. So, for clarity and brevity...

At what distance from DESTINATION should a pilot begin descent (TOD)?
Given...
- PERCENT OF TRIP DISTANCE (flight profile experimentation)
- AVERAGE GROUND SPEED
- CRUISE ALTITUDE
- DESIRED ROD
- DESIRED DISTANCE FROM AIRPORT AT BOD

Please keep in mind this "tool" is intended for inexperienced sim pilots to eliminate the guesswork in ascertaining the proper TOD location. The goal is to have any sim pilot of any experience who uses this tool to arrive at the BOD perfectly situated to enjoy a smooth transition to approach and landing.

My journey to a solid modicum of understanding was hard won, painful, fraught with frustration and littered with failed experiments where I took my "best guesses" I would make a whole series of guesses, test them out and see if I could find any patterns in the chaos. I never did which is why I am seeking help. After two years of trying with no real success, I figured it was time to send up a white flag. Thanks for your help.
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kullery
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Re: TOD Calculator for Sim Pilots

Post by kullery »

Hi Calvin,

You might want to check out a free I-phone app called Pilot Wizz. It has a number of very handy aviation calculators and it is very easy to use in the cockpit (RW and sim). One of the calculators is "rate of descent". It will perform these 3 calculations:

Determines distance, time, and gradient based on ground speed, altitude change, and rate of descent

Determines distance, rate of descent, and gradient based on ground speed, altitude change, and time

Determines time, rate of descent and gradient based on ground speed, altitude change and distance


It is available at https://itunes.apple.com/us/app/pilotwi ... ?mt=8&ls=1.

Good luck with your project.

Ken
Ken Ullery - PPL-SEL, 1G5
Orest Skrypuch
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Re: TOD Calculator for Sim Pilots

Post by Orest Skrypuch »

It is really pretty simple, you don't need an "app", you can do it in your head - and should. This is just based on the fact that a 3 deg slope is about a 300ft drop for every 1 nm downrange.

1) Plan to descend at a 3 deg slope, to do so descend at 5*GS. (eg. GS = 200 knts, 200*5 = 1000; descend at 1000 fpm)

2) For your top of descent, take your height above the airport, in 100's of feet, and divide by three. (eg. if cruising at 12,000ft above the airport, start your descent at 12,000/100/3 = 40; at 40nm)

Keep tabs on your ground speed during the descent and adjust as necessary, as the wind will generally decrease and back (shift couterclockwise) as you descend.

In a non-turbo small prop, do not change your power setting when you start your descent, accept the increased speed, just gently keep pulling back the throttle, to maintain the same MP. If you were well above 10,000, let the MP increase to about 20", then hold that. That will generally keep you below your Vno in most single engine props.

Adjust your pitch, to maintain your VS.

Incidentally, that is the reverse of what you will generally do on final approach in a small prop, where you want to keep your airspeed constant, but vary your descent. There you would keep your pitch relatively constant, and adjust the power to adjust sink.

* Orest
Last edited by Orest Skrypuch on Sun Jan 06, 2013 8:03 pm, edited 6 times in total.
PP/ASEL/IR, Piper Dakota (PA28-236) C-FCPO
President & CEO, UVA, http://www.united-virtual.com
Calvin Waterbury
Posts: 199
Joined: Mon Nov 07, 2011 10:37 am

Re: TOD Calculator for Sim Pilots

Post by Calvin Waterbury »

kullery wrote:Hi Calvin,

You might want to check out a free I-phone app called Pilot Wizz. It has a number of very handy aviation calculators and it is very easy to use in the cockpit (RW and sim). One of the calculators is "rate of descent". It will perform these 3 calculations:

Determines distance, time, and gradient based on ground speed, altitude change, and rate of descent

Determines distance, rate of descent, and gradient based on ground speed, altitude change, and time

Determines time, rate of descent and gradient based on ground speed, altitude change and distance


It is available at https://itunes.apple.com/us/app/pilotwi ... ?mt=8&ls=1.

Good luck with your project.

Ken
Thanks Ken. I'm sure your link will be valuable to the iPhone-ers, but I live in a Blackberry world.
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Calvin Waterbury
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Re: TOD Calculator for Sim Pilots

Post by Calvin Waterbury »

Orest Skrypuch wrote:It is really pretty simple, you don't need an "app".
...
* Orest
Hey Orest, thanks for the non-formula (and the bonus material). I'll put it through its paces and see if it fills my criteria. Actually, I have often wondered if there was an easier way?

Ciao!
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Re: TOD Calculator for Sim Pilots

Post by Keith Smith »

Calvin, if you google "top of descent rule of thumb", you'll have all the info you ever dreamed of regarding top of descent calculations. This is one of those cases where if you want to write an app, have at it, but the problem has been pretty well tackled in industry.
Calvin Waterbury
Posts: 199
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Re: TOD Calculator for Sim Pilots

Post by Calvin Waterbury »

Yeah, yeah...I know you can find anything you want on Google these days. Canned answers seems to be the rule of the day. It certainly does save time for people who have a full plate. Not criticizing individual lifestyles and I would be grieved if I found out I added to an already heavy load. I just thought it might be nice to see what multiple human beings might come up with in a relaxed and elongated discussion. Who knows what might have happened!?

I'm a dinosaur who came from the days where the place to exchange ideas was the water cooler, the rail fence and standing at the fuel pump at the gas station. I really wasn't trying to make an app, just give a noob something to start with regardless of what they are flying. Actually, I was aware there were many ROTs out there, but my intent was to hone the "rule" to a razor point. I hope I'm making sense.

No offense intended and I do appreciate the contributions. I'll grab a handful and see what rises to the top. :)
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Keith Smith
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Re: TOD Calculator for Sim Pilots

Post by Keith Smith »

That's a genuinely interesting outlook, Calvin. The exercise of producing a TOD rule of thumb would certainly be valuable for a lot of people and would get them thinking about flying in a way that they otherwise might not have done. I misunderstood, I thought you were saying you'd been wrestling, on and off, for 2 years of how to handle TOD's and that you were thinking of building an app do it. We just don't want to see you spin your wheels on a problem that's been solved many times over, unless you already know that it's been solved and you have other reasons for doing it.

If it helps, I try to look at the things that people tend to have trouble with and try to help build training material around those topics. Those are problems which 'need solving' IMHO.

What I might do is post a list of coding projects that could directly benefit PE that I can't tackle any time soon, and that can be developed and tested independently of PE. There are a number of those regarding radar and radio signal strength testing that I'd love to see happen, if those might catch your interest :) I sense you're itching to do SOMETHING...would that be right?
Calvin Waterbury
Posts: 199
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Re: TOD Calculator for Sim Pilots

Post by Calvin Waterbury »

You're not really wrong. Try not to read my ramblings *too* specifically. As far as your external app needs, you could probably code it faster than it would take for you to explain what you need and I am confident there are far better coders on this forum than myself, but I'm game to try. Ironically, I really wasn't looking to code an app this time. At least not at this point. I was really only looking for a formula, like what has already been offered, so I'm good for now. I really want to test out what has been offered before I pursue this thread any farther. Lastly, I want to rethink my last two years to see if I am looking at this whole thing the right way. I've been given some food for thought in this thread.
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Mark Hargrove
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Re: TOD Calculator for Sim Pilots

Post by Mark Hargrove »

Keith wrote:
What I might do is post a list of coding projects that could directly benefit PE that I can't tackle any time soon, and that can be developed and tested independently of PE. There are a number of those regarding radar and radio signal strength testing that I'd love to see happen
Please do post such a list! I "grew up" as a software engineer, still love to code, but don't get a chance to do it professionally any more. It would be a lot of fun to tackle an interesting problem that I'd have to learn about.

-M.
Mark Hargrove
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PE: N757SL (Cessna 182T 'Skylane'), N757SM (Cessna 337 'Skymaster'), N757BD (Beech Duke Turbine)
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