What to use for TOD
Posted by fullpower440@reddit | flying | View on Reddit | 20 comments
We know that to calculate the TOD we simply calculate how much altitude to loss/300, some others how much altitude to loss*3 + 5or10 (depends on the plane).
So, to reference how much altitude to loss you use the actual crz altitude down to where?
-Transition point from the star
-Airport elevation
-Intermidiate/final Fix
What you think is best here?
blindpylote@reddit
For every thousand feet to lose, start your descent 3 miles out. For rate of descent its ground speed divided by two, then multiplied by ten. 90/2=45X10=450 FPM another way to think about it is ground speed multiplied by 5, but I think the divided by two times ten way is easier on the fly.
Loud_Cream7702@reddit
This is the way.
propell0r@reddit
Altitude to loose in thousands * 3 then add 10 for the 250kt slow down. Set VS to half your groundspeed with an extra 0. Good enough for government work
I_ALWAYS_UPVOTE_CATS@reddit
~~Whatever the FMC says.~~
It's 3 miles per thousand feet above the aerodrome elevation*. This will get modified slightly if the arrival follows a circuitous route or has intermediate level restrictions. To avoid ending up high on approach, we plan our descent to allow for the best-case shortcut that could be granted by ATC, allowing for any level restrictions.
*There are other considerations as well. Needing to slow down will add miles, as will a tailwind during the descent. Conversely, a headwind will reduce the track miles required. There are rules of thumb for this (that I can't remember off-hand), but in practice there's a reason why I opened with that crossed-out sentence.
Asieloth@reddit
For me, personally, I generally reference the FAP/FAF orb IAP/IAF. So, use that difference in altitude.
JasonThree@reddit
You can't go wrong with 30 miles out at 10k. Above field elevation of course. Some guys get real crazy with airport -5 at this or that altitude or set the airport at 4000' if it's downwind. I just go along with it. But 30 miles at 10 is always good enough.
braided--asshair@reddit
If you’re on the departure side of the airport, just throw IAF alt and 210 knots on the airport and VNAV will take care of the rest. If you’re on the approach side of the airport, throw the speed and altitude 10-15 miles prior to the destination airport. Works really well like 70% of the time.
confusedguy1212@reddit
It’s all rule of thumb not exact science so you can use anything. You can also fudge along the way.
In jets it’s about 25-30 minutes from touchdown. Or 120NM miles from direct to the airport roughly.
LizardKing77733@reddit
3-1 profile, plus distance to lose speed at 10000 and 3000.
BrtFrkwr@reddit
Three miles a thousand to airport elevation.
AK_Dude69@reddit
And some Kentucky windage for head/tail winds and restrictions.
BrtFrkwr@reddit
Always. It's called situation awareness. Some of the older FMSs would put the T/D dot a little too far in so I would lie to it and tell it the wind was 100 knots on the tail so I would have a nice, gentle VNAV descent.
imblegen@reddit
It depends. What aircraft are you flying? What avionics do you use? What kind of flight is it? What procedure(s) are you flying? How high are you cruising?
My answer would change depending on all of those things.
zmenz1097@reddit
For airports with a star I just let VNAV do its thing. If there’s no star, I set a point 30 miles from the airport at 10,000 feet. That way you have a standard 3:1 descent. Of course for that you have to compensate for field elevation but where I fly it generally works out
davidswelt@reddit
Garmin nav unit (any of them)™ utilities, VCALC. Select waypoint, offset. MSL/AGL, and desired descent rate (unpressurized: 500 with pax, 700-800 without if.tailwind). To calc the offset I need to measure the distance from the target to the Bravo ring I want to stay under, oder set a waypoint there. Obviously looking at ETD and doing quick math is easy too :)
rotardy@reddit
Time
YugeWaterBottle@reddit
I usually ballpark 10,000 ft loss for every 30 miles. Anything around there will be close enough. If you're in the 30s, you probably want to start down around 100 miles from the airport.
2000 fpm is a good ballpark. Adjust as necessary.
RyzOnReddit@reddit
I put in a crossing restriction 3 miles before the airport and then wait for VSR to get close to what I feel comfort descending at.
NolanonoSC@reddit
Usually an approach plate will list out the minimum IFR altitudes when 100nm out, 25nm out, and at IAF. Depends on how high up you are but I usually plan a 1000ft/min descent and use a flight calculator to figure out how many miles out I should start descending based off ground speed. In an unpressurized aircraft I usually do 500ft/min
rFlyingTower@reddit
This is a copy of the original post body for posterity:
We know that to calculate the TOD we simply calculate how much altitude to loss/300, some others how much altitude to loss*3 + 5or10 (depends on the plane).
So, to reference how much altitude to loss you use the actual crz altitude down to where?
-Transition point from the star
-Airport elevation
-Intermidiate/final Fix
What you think is best here?
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