Dear friends followers,
welcome back to my channel and to a very important topic you are most
likely going to mess up at least once in your pilot career - descent planning. How does a pilot know when to initiate his
descent towards its destination aerodrome? This is today's question. So I'll show you a few thumb rules Which will make your descent a lot
easier and avoid coming in hot and high. So get your brain ready for some easy
head calculations and let's get started! [Music]
[ATC]
[Jet taking off] This is undeniably one of the
procedures every pilot messes up at least once in his pilot
career - - the descent planning. Tell me about it, transitioning on to the Boeing 747, which is heavy as lead and fast as a
bullet, will slap you in the face if you think you can fly her like an
Airbus A320, which has a glide ratio comparable to a flying piano
once you extend to speed breaks. There is only little room to play with
once you fly a wide-body airplane. But how is it done? Okay, the times where you flew
inbound to your destination airport and just look outside of the
window and then decided to descend visually towards the runway are over,
at least within the airline industry. Now you might have seen these abbreviations
and videos about a navigation display This shows the top of
descent point in regards to the entered flight plan in the FMGS on
Airbus models or on the FMS on the Boeing. Obviously, this is a great help showing you
when to start your descent, but there is a big flaw with that top of
descent point on the ND and I'll show you in an example why and why it is important to me that YOU know a few thumb rules to
NOT mess up your approach. Now, let's assume we're approaching
into Hamburg, Germany. Now we've got the approach set up, but as you see here on those two ND's, the top of descent points
are at different locations. How can that be? Now this ND shows a direct
approach towards runway 33, and the other includes a
so-called "transition". A transition is an approach procedure in order to slow down incoming traffic and makes it easier for the approach
controller to stack the planes, or as they say, "pushing tin", before handing them over
to the tower controller. So with a transition, the
flight or approach distance moves the top of descent to a later point, compared to a direct approach,
which will want you to descend earlier, as you only have a
certain distance left over. This makes sense, right? Compare it as if you were driving
to your girlfriend's house and she wants you to call her 30
miles prior reaching the house to get dinner ready. Now, you are on a direct
route, but let's say [that] you need to perform a little detour, or a Transition, at your
ex-girlfriend's house, that would move your time of call, or top
of descent, to a later point, right? So in order to anticipate that the controller
MIGHT take you in for a direct apporach, always plan with the SHORTEST
distance to the runway. Now, the easiest way of finding
out the distance to the airport is by tuning in the closest VOR DME
to the runway you're flying to. So in our case, Hamburg has a DME station
named Alster right at the aiport. So we're gonna be tuning into Alster (ALF)
and [it] will give us a distance of. Now, as you see on the PFD
and ND, we're at 27,000 feet with a speed of 320 knots,
and a tailwind of 20 knots. Let's neglect weight of
the plane for a moment. So with that data we want to
calculate the distance we need to fly a continuous descent profile. So the first thing we're gonna look at is
the altitude we're gonna have to lose. Now, Hamburg is at 53 feet above sea level, we can disregard that, so we effectively have
to descend 27,000 feet. Now, keep in mind in your airport
destination is Bogotá, for example, you would have less altitude to lose as
the aiport already is at 8,300 feet. So, we're gonna cross out the last
digits and divide that by 3, so 270 divided by 3 equals 90. So 90 nautical miles is the MINIMUM
distance we'll need for our descent. Now, the next factor to consider is speed, as there will be no way we'll be
landing the plane at 320 knots. Now we'll consider 200 knots
as an average descent speed as you are progressively getting
slower over the entire descent profile, including company speed restrictions
like 250 knots bellow 10,000 feet. But I'll come back to that in a minute. So we've got to lose 120
knots indicated air speed, not ground speed, and a good estimate for most planes is 1
nautical mile per 10 knots speed reduction. So, 120 knots divided by 10 equals 12
nautical miles to reduce the speed. Last, but definitely not
least, and often forgotten, but easily messes up your
descent planning is the wind. Now, in our example we have a
tailwind, which will make it harder for you to lose altitude and kill
speed, so for every 10 knots of wind you add 1 nautical mile, so we have 20 knots divided by 10 equals 2 nautical miles we
have to add to our milage. Now, if it were to be a headwind,
you SUBTRACT the miles. Capisce? So, summing up our calculations, we should
be starting our descent at 104 track miles. As you are in descent, you
want to check every 10,000 feet if you're still on the
correct vertical profile, and then decrease or increase
your vertical speed. Speaking of vertical speed, there
is another technique you can apply. Now, we'll use 200 knots as an average
descent speed, as mentioned previously, so we determine the distance
needed of 104 nautical miles, so if you use your
calculations that say 100 NM, that means you need 30 minutes
for the entire distance. 27,000 feet divided by 30 minutes
equals 900 feet per minute, which should be your
average vertical speed. I'm personally not a vertical speed
flyer, and I'll tell you why: first of all, 900 ft/min is in
most cases not steep enough for the intitial descent,
and the controller will be fuffing about why you're
descending at such [a] slow rate. In the rarest cases, the vertical profile
is a 3° glide slope from top to bottom, as it is sheer impossible to maintain that
slope, and reduce speed at the same time. Compare it as if you were coasting
downhill a highway at 60 miles, if you wanted to slow down,
but you couldn't use the brakes. How would you slow down? The only way you'll be doing that is
if the highway would become shallower. So, commonly, pilots start up with a
high speed descent at 3° or steeper. Now, just before reaching 10,000
feet, you dial down the speed, the plane pitches upwards,
creating a shallower descent path, equal to 1,000 feet per 5 NM. Now, once you receive for the descent
clearance, you rejoin the 3° angle, equal to 1,000 feet vertically,
needing 3 NM horizontally, maintain 250 knots until
reaching 5,000 feet, dial down the speed even more so
you can extend the slats and flaps to establish yourself on the
ILS, VOR approach, etc., so your vertical profile is going
to look something similar to this. Now once established on the
approach, another thumb rule - ground speed times 5 gives you the
necessary vertical speed feet per minute you want to be flying to
maintain on the 3° glideslope. But for the descent I primarily use the
first three thumb rules I've mentioned, and your goal should be, as it is mine, to fly the entire
descent on idle power and ONLY apply thrust after
the final flap setting. The least efficient thing you can do is if
you use the speed brakes for the descent and then you have to apply thrust to maintain
altitude before reaching the glideslope. You've then killed kinetic
energy for no good reason. That's the point I'm at right now
with the Queen [Boeing 747], so she's still testing me, but
I have to admit that sometimes we land with the maximum landing weight and
the next day we land with 100 tonnes less. And that for sure has an impact on the handling
and descent characteristics of the plane. Let's look at a scenario
which is quite common: the controller won't give you the
descent clearance due to traffic crossing at lower levels. So you slowly get over
your descent profile. Now what? Now, there are three things you
can do to rejoin your profile once you have your descent clearance: increase speed and indle power; dial up the speed and pull
back the thrust levers and the autopilot immediately will try to capture the speed, but without thrust, and the only way of doing this is by pitching down, so you and your passangers will feel that
descent, believe me, but it's fun though. Try to get below your descent
profile as quickly as possible, as that will give you space and time
to slow down the airplane again. Now, if ATC advises you to
maintain a certain speed, which is too slow to kill altitude, use your speed brakes - the airbrakes, the high drag devices - whatever
the manufacturer named them, that's what they are made for. A video about that topic is coming up soon. Now remember the car
scenario mentioned before. Now, you're allowed to use the
breaks actually to slow down. Now, you might even use the gear or flaps, but you're most likely not gonna
retract them once you've used them. And the last resort, ask for delay
vectors or more track miles. You ever so often hear
collegues ask the controller: "How many track miles can we expect?", and will response, let's say, "25 nautical miles", the pilots quickly do their
calculations, and if they're too high, they'll say
"Unable, request delay vectors". So the controller will extend their downwind,
for example, to give them more time to lose altitude or reduce speed. Now, you might say: "Joe, this is all nice to know, but I'd rather
rely on the data I've put into the FMGS, or FMS, and I use all the help given
on the navigation display." That's fine by me, but the FMS and
all the other help cannot anticipate a sudden direct approach given by the
controller, and if you're replying "Unable, we're too high for the approach.", you'll be the reason for many smiley
faces in surrounding cockpits, and they'll be happy to nick
your slot for the approach, so fly conservatice, but also plan ahead, constantly monitor your distance
to the airport and, if necessary, use the speed brakes or extend
the flaps, or even the gear, to lose speed and altitude. As always, situational awareness is KEY! You'll get a feel for it, don't worry. Also, on how to anticipate
the surrounding traffic and the mood of the air traffic controller. A go-around [be]cause you're
hot and high might happen as you're unable to establish
yourself on the approach, but it's not a failure. It's more of a lesson
that you'll profit from. That's it for today, thank
you very much for your time! To become my wingman, hit that subscribe
button and that notificaiton bell, so you won't miss out upcoming videos. And don't forget, a good
pilot is always learning! See you next week, all the best, your Captain Joe! [Outro] [Outro]
[English subtitles transcribed by Iliyan Angelov]
Generally for descent planning, you want to be at 10,000’ AGL around 30nm from the airport. You can use the 3:1 rule meaning that if you’re at 30,000’ and need to descend to 10,000’ you’ve got 20,000’ to lose. Knock off the “thousands” to get 20 and multiply by 3. That gives you 60, so 60nm from where you want to be at 10,000’ you should start your descent and this will provide a 3-degree path at around 333 feet-per-mile to meet that point.
If the airplane has an FMC and VNAV, it will calculate the descent based on airspeed and winds while also calculating the descent path required to meet any crossing restrictions that may be on the arrival. If there are no crossing restrictions, then we can place a point which is usually around 30nm our from the field to cross at 10’000’ AGL. Alternatively we also sometimes tell the FMC we want to be over the field at 3,000’ AGL...this does the same thing as the 30nm point, it’s just a different technique.
I hope that makes sense.
Whenever I want that’s when!! Seriously I just use the fms when it gives me the descent rate I want I start down, usually 2000-3000fpm and 320knots from cruise altitude, Ill usually adjust for turbulence and running the anti ice system. I don’t do mental math it’s too much work.
If I'm 5 miles until the IAF, with the IAF altitude at 5,000', and I'm at 10,000', I'm descending at 1000'/min. Simple as that (for an aircraft not equipped with an FMS or other similar system).
I wrote a semi-lengthy reply to another comment, but to answer your main question, you *may* have to do math in your head if you haven't been able to plan your approach in advance. A good pilot will be looking at all the approach plates for where they're going on the next flight and calculating all this stuff while on the ground. I use ForeFlight and I just write my descent rates for different approach segments right on the charts. All I have to do is read them off. Of course, if you have to divert or just end up going somewhere you hadn't planned, then you may be looking at approach plates for the first time and will have to calculate in the plane.
Same for top of descent. If your EFB or avionics don't calculate that for you (though you should really know how to do it), you should already have a good idea anyway based on the flight planning you did. When I was in training, I'd get this *super* exact based on ground speed with winds and power settings taken into account so nobody could call me out on it, but in the real world you can just use rules of thumb because everything gets kind of adjusted as you go along anyway. But you should at least start with numbers that are reasonably good; we want to adjust from good numbers so we're making small adjustments. So you still want your estimates to be pretty close. That kind of comes with experience.
In an airline setting, or even on an IFR flight plan in GA, you're going to be told by ATC when to descend, but you still should know around where you need to start down in case they forget you. I've had that happen; then you just call and request descent. But you can't do that if you don't know where your descent needs to start.
But yes, pretty often you need to do math in your head. That was one of the hardest things for me to learn; even simple math is hard to do in your head when you're hand-controlling an airplane. (It's a little easier with an autopilot.)