Lesson 2: Energy Management
Imagine sitting on the back of a mean rodeo bull as
the gates open and the audience cheers. You're holding
on for dear life, ruing the day you gave up being a
vegetarian. The stakes are high because the big steak
you're riding is in rare form and in a mighty bad mood.
After what seems like hours of trying to steer the
steer, you're ejected from the beast and your life
saved by rodeo clowns.
What does this have to with flying a jet? Without a
good understanding of energy management, you'll
probably feel something similar to riding a wild bull
when trying to get a jet down and stopped on a runway.
The only thing missing from this scenario will be the
rodeo clowns. Like riding a steer, controlling a jet
means understanding how to manage its
energy.
Descents
We have covered the fundamental points involved in
getting you airborne and leveled off at cruise
altitude. Now you're wondering how you can descend and
be in the right place at the right speed and altitude
for landing. When it comes time to begin the descent,
you must complete several important tasks in order to
end up at the right place and at the right time. Prior
to actually beginning the descent, air crews must
complete the following:
- Plan when to start the descent.
- Get the ATIS and other information relating to
the approach and landing.
- Calculate (or estimate) the landing weight of the
aircraft.
- Determine the flap setting and Vref speeds for
landing.
- Determine the appropriate landing runway and
approach desired.
- Brief the crew on the specifics of the
approach.
- Complete the Descent checklist.
Descent Planning
Descent planning is not a trivial matter. It
requires some expertise—but we will show you the
basics required to get the most enjoyment from your
Flight Simulator experience. So here's the lowdown on
getting down.
Aircraft equipped with turbine engines achieve their
best performance at high altitudes where the air is
thin and fuel consumption is low. Jets like to climb
high quickly, and remain there for as long as possible.
Although it may be better for the airplane's
performance, a steep high-speed descent is not the most
enjoyable experience for passengers. So, alas, we must
find a way to descend from altitude at a more
comfortable rate. After all, during the descent the
last thing you want your passengers to do is throw
their hands over their heads like they would on a wild
rollercoaster ride, right?
Air crews flying large jets at high altitudes like
to descend using the 3:1 principle. That is, for every
3 nautical miles (nm) traveled forward, they descend
1,000 feet. This rate produces the same 3-degree glide
slope you typically use on final approach. There are
two key components to descent planning: 1) When to
start descent, and 2) Which rate of descent to use.
When Do I Start My Descent?
To figure out when you should start down from your
cruising altitude, you need to know the point and the
altitude where you'd like to be along your route. For
simple VFR flight, you might want to be at traffic
pattern altitude (1,500 AGL) as you enter the pattern.
Commercial airliners often must follow an IFR Standard
Terminal Arrival Route procedure (or STAR—which,
by the way, isn't only used at night) that dictates the
altitude and speed at which you need to be at points
along the specified route. For example, a STAR might
request that you descend to cross the 30 DME fix from
the ABC VOR at 10,000 feet and maintain 250 knots (I
guess we could call this a *falling star*, but we
won't).
To figure out your starting point, or "Top of
Descent," while following the 3:1 rule, subtract your
target altitude from your current altitude and multiply
this answer by three. This will give you the number of
miles from your target that you need to start your
descent. Consider the following examples:
If we are flying at 8,500 feet and want to be at
1,500 feet at the airport, calculate your descent point
as follows:
8.5 |
Our starting altitude, in thousands of
feet |
-1.5 |
Subtract our target altitude, in thousands of
feet |
----- |
7.0 |
Our altitude to lose, in thousands of feet |
x 3 |
The magic 3:1 multiplier |
----- |
21 |
Nautical miles from the airport |
Here's another simple way to calculate this in your
head. Once you know the altitude you want to lose in
the descent, multiply it by three to determine the
miles from the fix you should be. We'll use this during
the lesson to monitor our progress during the descent,
which will then tell us if we need to make adjustments
along the way.
According to our STAR instructions example above, if
we were cruising at 28,000 feet and needed to be at
10,000 feet 30 miles from the VOR, our calculations
would be as follows:
28 |
Our starting altitude, in thousands of
feet |
-10 |
Our target altitude, in thousands of feet |
---- |
18 |
Altitude to lose, in thousands of feet |
x 3 |
The magic 3:1 multiplier |
---- |
54 |
Nautical miles from our destination point |
Here comes the fun part of this example (and no
rodeo clowns are involved). We need to begin our
descent 54 nm from our target destination
point—the 30 DME fix from the ABC VOR. This means
we need to add in the additional 30 nm to this 54 nm
figure and start our descent 84 nm (54 nm + 30 DME)
from the ABC VOR.
You can have a lot of fun calculating descent points
and then flying them to check your assumptions. The
next important element of the equation is the actual
rate of descent you use during the descent profile.
What Rate of Descent Will Get Me
There?
Using the right rate of descent in feet per minute
(fpm) will be the key to arriving at your target
destination at the proper altitude. In order to
maintain our 3:1 descent profile, we can calculate the
required rate of descent by multiplying our estimated
ground speed for descent by six.
But how will you know what your ground speed will be
in the descent before you descend? Our profile initial
descent speed from altitude will be approximately 0.74
Mach (or about 400 knots ground speed). So, you can use
400 knots as your initial ground speed during descent
and calculate the required rate of descent by
multiplying 400 (knots) by 6 to get 2,400 fpm.
So, in our example above, at 84 nm from the ABC VOR,
begin a 2,400 fpm descent.
In cruise, you have been traveling at around 400
knots with N1 set at around 71 percent. For descent,
initially reduce your N1 to 60 percent to 62 percent to
keep from over speeding the aircraft.
A very quick way to estimate your descent rate is to
consider the following:
Above 25,000 feet |
300 knots at 2,500 fpm |
Below 15,000 feet |
250 knots at 1,700 fpm |
200 knots at 1,400 fpm |
How Fast is Too Fast?
A subtle piece of the equation is speed control.
There are two places you will need to modify your
profile to keep within parameters: during descent as
you descend into thicker denser air, and at the level
off point where you may need to start slowing down to
meet any assigned speed restriction (for example,
slowing to 250 knots).
Now, it's very important to remember the FAA
regulation that limits airspeed to no more than 250
knots when you are below 10,000 feet. In the lesson,
we'll stay below this airspeed, but it'll be important
to remember on your checkride.
As you descend into thicker air, your airspeed unit
of measure will change from a percent of the speed of
sound (Mach) back to knots. To prevent a close shave by
flying too fast when it's not appropriate to do so, you
can determine this threshold by noticing the "barber
pole"—the red and white striped pole, or needle,
displayed on the upper left side of the airspeed
indicator. This needle marks the "never exceed speed"
for the aircraft. During descent, the barber pole
increases toward the needle of the airspeed indicator
and left unattended, will eventually cross paths.
Should this happen, you will have an over-speed
condition on your hands as marked by the
"click-clacking" audible warning, which also sounds
like the copilot's chattering teeth because he's quite
nervous by now. To avoid this, reduce N1 to 45 percent
and maintain 310 to 320 knots during the remainder of
your descent.
As you descend from cruise altitude, you will be
storing up all kinds of kinetic energy as you go
swooping down at more than 300 knots. All this works
against you as you reach your target point and need to
slow down. The solution is easy. During your descent
planning, allow for an additional 5 nm to level off and
slow to your target speed at idle. In our example
above, this would mean beginning your descent at 89 nm
from the ABC VOR, arriving at 10,000 feet 35 miles from
the VOR. Once leveled at 10,000 feet, reduce power to
flight idle and coast the 5 nm bleeding off speed until
you reach 250 knots. At this point, increase throttles
to N1 of 52 to 55 percent and maintain 250 knots.
And now a word of warning from your sponsor--me. If
you have been using the autopilot with autothrottles
enabled, you may not slow down in time to meet your
crossing restriction. In the second example above, the
autothrottles will get you slowed to 250 knots, but not
by the 30 DME fix. Typically, the autothrottles can
take as much as 7 minutes to slow down. You can cover a
large distance in this amount of time. Without access
to the Flight Management System (FMS), the best way to
handle this is to disengage autothrottles as you level
at the 5-nm lead point and set power to flight idle
manually. Optionally, you can use a 10-nm lead point to
account for the extra time and distance required for
autothrottles to slow down.
Of course, as a last resort, you can always deploy
the spoilers (or speed brakes) by pressing the **SLASH
(/)** key. With careful planning, such as the above
steps, you should be in fine position for your approach
and landing profiles.
Now you are an energy manager because the 3:1 rule
on descent planning is true for any plane you fly, such
as the Cessna Skyhawk and Beechcraft Baron 58, and at
lower altitudes and airspeeds. Perhaps I should now
call you the *all-being, master of space, time, and
dimension* because so few people ever master energy,
much less master it in a Boeing 737. OK, I won't call
you that yet. All I'll say is give this lesson a try
and practice the principles you've learned here. More
power to you!
Ok, see you in the cockpit. Click the **Fly This
Lesson** link to practice what you've just
learned.
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