Altitude and Speed constraints during STAR (or Approaches)
It is imperative to understand how FSX GPS works to properly fill Atitude and Speed constraints cells in the EXCEL datasheet.
There are two particular concepts to always keep in mind:
- The « Active » Waypoint is always the next waypoint in the flight plan ( « Target » Waypoint)
- FSX anticipates arrival to a Waypoint, with a distance according aircraft ground speed. This distance can have a range from 0.2 Nm at low speed, almost 4 Nm at high speed. This has the consequence that the target Waypoint of your flight plan flight plan becomes active (‘and therefore all data related to it in your EXCEL form), before you have crossed the previous Waypoint.
This mechanism has implications about speed constraint concept that is related to the passage of a waypoint on your STAR or Approach chart. While on the chart, speed constraint is specified for a particular waypoint, it means that it should apply from the waypoint.
Imagine a STAR with 3 Waypoints: « A », « B » and « C ». Imagine that our STAR has a speed constraint at the waypoint « B » 250 IAS and altitude constraint of 12000 ft, which means two things:
- You must not go below 12 000 ft long as you have not exceeded the waypoint « B »
- You must be a maximum of 250 IAS from Waypoint « B »
But in our aircraft, as soon as you started your descent, « B » waypoint will become the « Active » waypoint once you have crossed « A » waypoint (even 2-3 Nm even before reaching « A », because FSX GPS anticipation phenomenon. so if you have registered waypoint « B » altitude and speed constraints in the EXCEL sheet, the 250 IAS speed constraint will be applied from the waypoint « A « as soon as you crossed » A « , because » B « will become the » active « waypoint.
As much this phenomenon is perfectly adapted to altitude constraints (since you should not go below 12 000 ft) unless you have not reached « B », as this may prove to be very embarrassing about speed constraints; Imagine you have a large distance between « A » and « B », then the aircraft will travel that distance at low speed with no reason, since the constraint is mandatory only from « B ».
So in this case, we must shift the speed constraint of one waypoint (set speed constraint to waypoint « C »), because « C » data will be applied to the FMC when Aircraft reaches « B », but since it there is the phenomenon of anticipation associated with ground speed, « C » will become active 2-4 Nm before aircraft reach it, so you have time to slow down, if necessary with Speed brakes.
If distance between « A » and « B » is very small, so it’s not worth it to offset the speed constraint, for precisely this distance will slowdown area.
Warning: This optional « constraint » offset only applies speeds! Altitudes must be at each waypoint, according to the chart, never stagger!
It is also for this reason that we must never set speed constraint for the first waypoint of a STAR.
To give a real world example, here, a LFPG – Charles de Gaulle, France STAR chart :
You can observe that « KOLIV » Waypoint has a speed constraint of 250 IAS and no minimum altitude constraint. For the altitude constraint, we will put the next waypoint altitude which is very close, or 10,000 ft.
But about speed constraint, we will indicate different values, according where we come from :
- STAR that come from Dieppe (VOR-DPE) or Deauville (VOR-DVL) include a waypoint named « SOKVI » located 10 Nm before KOLIV. So in this case, we can set the 250 IAS speed constraint at « KOLIV » because it will apply as soon as Aircraft has passed « SOKVI » but as he is left with only 10Nm to cross, this will slow without penalizing the flight time .:
- By cons, with STAR from Caen (VOR-CEN), you can see that there is a distance of 34nm between ‘KENOP « and » KOLIV « ; So if we enter the EXCEL sheet the constraint of 250 IAS at « KOLIV », this speed constraint will be applied as soon as the aircraft has crossed « KENOP » (since it is at this point that « KOLIV » becomes the flight plan active waypoint ), so aircraft have to travel 34 Nm separating « KENOP » of « KOLIV » at 250 IAS, while chart not require this constraint !. It is in this case it is imperative to shift the speed constraint to the next waypoint, so « MOPAR ».
This principle can also be applied to approaches files (I will do an article that is related to this phenomenon, regarding the speed to declare at the FAF).
Well, I hope I have cleared this speed constraint issue that raises many questions.
May 02, 2015