![]() If we are flying the approach at 180 knots, we need a VS of approximately 1000 fpm (5x180) reducing to a VS of 600 fpm when we are flying at 120 knots (5x120). It also helps to know what VS is required to fly a 3 degree final approach slope. Though we may not be in VNAV for all altitude step downs (eg on radar vectors), ATC expects an appropriate vertical speed, and rounding to 1000 fpm matches the 3 degree profile closely. our target VS would be about 1200 fpm (5x240). This logic carries through the arrival phase where we slow to a GS of 240 knots. Remember, you must be in LNAV first (instead of heading) so that the FMS has a starting point for the calculations. We set the desired VS (normally 2000 fpm) and the VNAV function will accurately determine our revised TOD distance and then try to maintain a VS as close to 2000 fpm as possible. This is where the VNAV function of the FMS makes life easier for us. To better understand, check out the image at the end of this article.Īs we get significant winds changing our groundspeed, the results for our VS calculation will change. If we descend at 2000 fpm, the 3x distance calculation will be valid. Assuming no wind, let's say a regional airliner flies at a GS of 400 Knots. If we go back to our example of crossing HNK-10 from cruise, we first look at our FMS or MFD for the current groundspeed. Again, there is a rule of thumb, and it is 5x your Groundspeed (GS). Let’s look some more at what VS is required to maintain the standard 3 degree slope. This is known as VTO, or Vertical To, the same way as DTO on a GPS means Direct To. The displayed Vertical Speeds (VS) are also helpful to let you know, if you started descent right now, what VS would be required. The FMS VNAV page will show you the slope and vertical speed between each waypoint on the screen. ![]() In these cases the FMS does a little extra math to determine the slope between the two points (it’s not necessarily always 3 degrees). It then repeats this process for all crossing altitudes you input into the VNAV system (such as a complex STAR). When you enter HNK-10 into the FMS, it does the above math for you, and advises when you are at the Top of Descent (TOD). The challenge is in knowing when you are a total of 28NM from HNK to start the descent – that’s why we have GPS and FMS (and in some cases, DME). Therefore, you would start descent 18NM from HNK-10 (3圆). If you need to cross 10NM prior to the HNK VOR at FL180, and you are arriving from FL240, you need to descend 6000’ (24-18). Suddenly, complex STARs become simple to fly. Therefore the descent would begin 69NM from the airport (3x23). If the airport were at 2000’ ASL, you would only need to descend 23,000’ (25,000 – 2,000). The easiest way to “ballpark” a distance back is through using the 3x table.įor example, if you are flying at FL250, and need to descend to an airport at sea level, you would start your descent approximately 75NM away from the field (3x25). ![]() ![]() The goal is normally to descend at a 3 degree slope (either from cruise, or an approach to landing). However, at higher altitudes with more complex STARs (Standard Terminal ARrivals) you need to be more precise – this is where VNAV planning comes in.Īs a review, VNAV begins with simple equations you can apply without any technology. When flying at lower altitudes in simple airspace, you get a “feeling” for when you need to descend from cruise. If you are not navigating to a point (GNSS waypoint, VOR station, DME fix, etc) you cannot use VNAV because you wouldn’t have a point in space to be working towards (you would be flying a heading indefinitely). In its simplest form, VNAV is how we navigate vertically – very much like how we use GPS or VOR to navigate laterally (LNAV). VNAV is a very useful tool in aircraft that have a Flight Management System (FMS) - in fact, we can even set up VNAV in many stand-alone GPS units! But before a pilot jumps into programming their VNAV, it's first important to look into, and understand, what VNAV is.
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