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.
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.