Vertical Navigation in G1000 MFD allows planned descents at a chosen slope angle to a height at a chosen way point. Great feature, but I want to add an audible warning that top or bottom of descent is a short time away. G1000 does not allow you to arm the VNV status for longer than (I think) 6 minutes and will turn it off after that. Very annoying if you are just about to reach TOD. So another reason to add an audible warning. Of course we need throttle changes on starting or ending a descent so it is important not to miss them. Sounds simple and I searched for a dataref that would tell me the time or distance to TOD or BOD. I found sim/cockpit2/radios/indicators/fms_distance_to_tod_pilot but on the Embraer Phenom 300 this does not give any value. (I suppose it only works on airliners).
Could a kind person help me to find a dataref or some method to know the time or distance to TOD and/or BOD??
Many thanks in advance
Anthony
Datarefs for Top and Bottom of Descent
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AnthonySullivan
- Posts: 93
- Joined: Wed Jul 01, 2020 8:06 am
Re: Datarefs for Top and Bottom of Descent
Isn't that x feet away from your set altitude?
Then you can either use an absolute difference, or calculate the time with your current vertical speed.
Then you can either use an absolute difference, or calculate the time with your current vertical speed.
Re: Datarefs for Top and Bottom of Descent
TOD is actually and obviously variable and depends on the type of aircraft.
For Airliners only, it goes like this, because for fuel consumption purposes the descent is done ideally on idle thrust, which equates more or less to a 5% descent slope. So the basic calculation goes like this: (Cruise Altitude - Airport elevation altitude) /300 = distance to TOD in Nm.
Example: CRZ fl290, Airport elev 2000 yields to 27000 ft to loose, hence BOD is 90 Nm (27000/300) from the airport.
To this initial value, one has to add the constraints of altitude in the STAR, and also the speed constraints below fl100 (usually 250 kts or lower).
- An airliner usually descend at +/-300 kts but it cannot descend AND decelerate at the same time, so once reaching Fl100 it will need a couple of extra miles to slow down from 300 to 250, usually 1Nm to loose 10kts. So add 5 Nm to the initial value.
- If the headwinds or tailwinds are strong, the Ground speed will be vastly different from the TAS. The idle slope respectively to the ground will be shallower or steeper. Add or substract 1Nm per 10 kts HW/TW.
- If the aircraft is light, it will not go as fast as expected in idle and its glide angle will be shallower, if the aircraft is heavier than its average weight (ie close to MLW), it will go faster and descend with a steeper angle. Allow 1Nm extra for 1Ton above the average weight (roughly 58/60 Ton).
This calculation should give you a TOD distance right in the ballpark.
For a Turboprop the calculation can be different because in Idle, the speed can bleed off more quickly due to the props. So the TID will be closer to the airport.
For an engine prop aircraft it’s even more different because you can rarely descent all the way with your engines at idle due to thermal issues.
Once you know the TOD distance, you can figure the Time to TOD, using you current GS
For the Phenom you mileage may vary, but I guess that apart the weight factor which can be disregarded, the rest is applicable, with probably less impact of the level off deceleration segment (I would probably use 2/3 Nm to bleed off the extra 50 kts instead of 5Nm)
Hope this helps.
For Airliners only, it goes like this, because for fuel consumption purposes the descent is done ideally on idle thrust, which equates more or less to a 5% descent slope. So the basic calculation goes like this: (Cruise Altitude - Airport elevation altitude) /300 = distance to TOD in Nm.
Example: CRZ fl290, Airport elev 2000 yields to 27000 ft to loose, hence BOD is 90 Nm (27000/300) from the airport.
To this initial value, one has to add the constraints of altitude in the STAR, and also the speed constraints below fl100 (usually 250 kts or lower).
- An airliner usually descend at +/-300 kts but it cannot descend AND decelerate at the same time, so once reaching Fl100 it will need a couple of extra miles to slow down from 300 to 250, usually 1Nm to loose 10kts. So add 5 Nm to the initial value.
- If the headwinds or tailwinds are strong, the Ground speed will be vastly different from the TAS. The idle slope respectively to the ground will be shallower or steeper. Add or substract 1Nm per 10 kts HW/TW.
- If the aircraft is light, it will not go as fast as expected in idle and its glide angle will be shallower, if the aircraft is heavier than its average weight (ie close to MLW), it will go faster and descend with a steeper angle. Allow 1Nm extra for 1Ton above the average weight (roughly 58/60 Ton).
This calculation should give you a TOD distance right in the ballpark.
For a Turboprop the calculation can be different because in Idle, the speed can bleed off more quickly due to the props. So the TID will be closer to the airport.
For an engine prop aircraft it’s even more different because you can rarely descent all the way with your engines at idle due to thermal issues.
Once you know the TOD distance, you can figure the Time to TOD, using you current GS
For the Phenom you mileage may vary, but I guess that apart the weight factor which can be disregarded, the rest is applicable, with probably less impact of the level off deceleration segment (I would probably use 2/3 Nm to bleed off the extra 50 kts instead of 5Nm)
Hope this helps.
My YouTube Chanel on the A320 (Real SOPs by an Airline Pilot IRL):
https://www.youtube.com/playlist?list=P ... 0Q6SBASRqJ
https://www.youtube.com/playlist?list=P ... 0Q6SBASRqJ
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AnthonySullivan
- Posts: 93
- Joined: Wed Jul 01, 2020 8:06 am
Re: Datarefs for Top and Bottom of Descent
Hello Ralph and JackZ
Many thanks for your detailed responses.
This confirms what I feared that there are no data refs available for time to TOD or BOD as displayed in the MFD of the G1000.
I have considered alternative methods of calculating the values from other information available but I cannot even find a dataref for the altitude of the waypoint being used as a target or the slope angle.
So, over to the method kindly proposed by Jack Z. It is not hugely important but I find audible warnings so helpful that I tend to add my own to make sure I don't miss something. This should be quick and easy if those data refs were available. Never mind.
best wishes
Anthony Sullivan
Many thanks for your detailed responses.
This confirms what I feared that there are no data refs available for time to TOD or BOD as displayed in the MFD of the G1000.
I have considered alternative methods of calculating the values from other information available but I cannot even find a dataref for the altitude of the waypoint being used as a target or the slope angle.
So, over to the method kindly proposed by Jack Z. It is not hugely important but I find audible warnings so helpful that I tend to add my own to make sure I don't miss something. This should be quick and easy if those data refs were available. Never mind.
best wishes
Anthony Sullivan