Autopilot - Friend or Foe - By William "BBall" Ball - Page 2 of 2
Before we start talking
about all the "bells and whistles" on Janes F/A-18's flight guidance
system, one important note about autopilots in general. They are obviously
just mechanical extensions of our gray matter, ask them to do stupid
things and they'll do it (again, I'll reference the airliner in the opening
paragraph). Have a good idea of what you will be trying to accomplish
BEFORE you engage it. Another very important point (they hammered this
into us in 757 ground school), just because you THINK you have the autopilot
the way you want it, and you've engaged it, be sure and check the annunciations
to confirm that it's doing what you've asked it to do. On the 757 our
annunciations are on the ADI, in our sims we'll most probably find them
on the HUD or the autopilot control panel; and in Jane's F-18 we get
HUD, A/P panel, and MDI (in the H S I mode) info.
O.K, what will the Jane's F-18 A/P-flight guidance system do for us?
First, let me say that it won't do anything for you it you're inverted,
doing a 5G Split S. Like all autopilots it has limits, and its limits
are: pitch less than 45 degrees, and bank less than 70 degrees. It also
has some speed limits. After doing some old fashioned flight testing
(aircraft wt.=145000 lbs, all altitudes are MSL, all airspeeds are IAS),
here are the numbers for the minimum speed to engage the autopilot; 15K=
200 kts, 10K= 215 kts, 5K= 235 kts, and 1K= 247 kts.
Now, let's break it down to the two planes of motion: vertical and horizontal.
We'll start with the vertical. What does every fighter/bomber pilot want
in the vertical modes available on an autopilot? A Vnav climb, cruise
and descent, saving fuel to keep the owners of the jet rolling in dough?
Hell no, that's airliner crap! We want to be able to hold an altitude,
be it 10,000' or 100', and we want to be able to fly a glideslope for
a precision instrument approach. Will this one do these things? Yep.
It will hold a barometric altitude (a trick for sneaking up on the tanker),
a radar altitude for those "low level" profiles (remember, this is NOT
a true terrain following radar mode….be careful with it when you are
really skimming the weeds), and it will couple with the glideslope (a
bit more on that later). It has another often overlooked mode that's
called FPAH (Flight Path Angle Hold), which basically keeps the jet at
the same pitch angle it was when the A/P was engaged. Do I use this?
You bet I do; it's great for those long climbs to altitude (or descents)
when you may want to be "heads down" setting up for the task at hand.
A word of caution; don't engage it at a high pitch attitude (I suggest
less than 10 degrees), for it won't give a rat's rear end about minding
your speed for you, and you could find yourself at a high altitude, a
high pitch attitude, and a slow airspeed…..never a good combination.
Try it, I think you'll like it.
What about the horizontal world? What do we want there? We want it to
be able to fly a heading, track to a navigation fix, track to a target,
track to a GPS fix, and track a localizer course. Of course if it could
do all this, AND possibly auto-land the thing when you're feeling like
the guy in the opening scenes of "Top Gun" (you know the one, he's sweating
bullets, rattled to the enth degree, and Maverick has to talk him down),
then that would be a BIG plus. Hey, guess what? This little jewel will
do all of this. As far as tracking things, it will track to the TACAN,
but it won't track a selected course. This means simply, that when you're
doing the "marshal stack" thing, you'll have to use "heading hold" to
fly the TACAN radial the babe in Marshall has assigned to you. No biggie.
It also has a Roll Attitude hold option; much like the FPAH, only in
the horizontal plane of motion. Couple this with the FPAH, and you have
something akin to a mode of the autoflight system in the 757; it's called
CWS, or Control Wheel Steering. In CWS, the autopilot is flying the airplane,
but you are "hand flying" the autopilot through inputs with the control
wheel. Basically, it allows you to set a pitch and roll attitude with
the wheel, and let the autopilot maintain it. When do we use it? Try
never. Needless to say, the Roll Attitude hold button doesn't get pushed
much in my F-18 virtual cockpit, but it's there if you decide to use
it. To quote a disco icon, "it's your thang, do what you wanna do."
So you launch, and are heading inbound to the target, what might be ONE
way to program the A/P to handle this mission? (These are certainly just
suggestions; take them for what they are worth.) Again, FPAH while you're
climbing outbound. It'll keep you in that profile while you do your "house
keeping" chores getting the cockpit set up to accomplish the mission
(I tend to set "bingo" fuel, jettison modes, initial autopilot modes,
and some others while still on the CAT….obviously, everyone will have
a different flow to get these things done). So vertical in FPAH, and
horizontal in coupled/waypoint. Level off, vert.now in BALT to hold your
barometric altitude. If it's a high profile, cool, but let's say that
the SAM threat is ugly, and we need to go in low….FPAH again back down
to 100' (or whatever) and RALT to hold pitch. Again, don't think that
it'll terrain follow for you, I make sure and set my RALT "alert" to
somewhere below my current RALT to give me a verbal jolt if it starts
to take me too low. Now comes the problem of which horiz. "path to glory"?
Remain in "waypoint", or switch to "TGT" if locked, or even "GPS" if
it's that's the target/ordinance du jour. Obviously your pre-flight planning
will play a big part in this, i.e., SAM threat, etc. You're the Capt.
of the ship, you decide.
You've done the deed, are target outbound and want to forgo all the waypoints
and just get home quickly. Pull up the Ronald Reagan TACAN, select TCN
on the H S I, couple it up, and away you go. No TACAN today on the big
"RR"? You say you don't want to use the waypoints in WPT CPL, or the
ACL function with "Heading Hold"? Well, then, it's divert time. Easy
as using the divert field's TACAN, and doing the same thing. One thing
to consider, most of the times I've diverted, it was because the jet
had holes in it and fuel was a BIG factor. Once you get the TCN and BALT
online for you, calling up the FPAS page is not a bad idea. Again, if
you had an autothrottle system and could select "long range cruise",
etc. then you wouldn't need this info (much). But you don't have that,
so you have to be the "long range cruise officer". Merely adjust your
speed until the "TO 0 LB" range doesn't read less than your TCN distance
to go. Also remember, the higher you can fly, the more fuel-efficient
those engines are going to be. This is a broad-brush statement, and make
sure you have the fuel to make that climb before you do it…won't do you
any good to use a lot of thrust to "go high", piss away a huge amount
of gas doing it, and now not have enough to make the divert field.
So you say that it's been a bad day? The world has gotten your panties
in a wad, the jet is full of holes, the weather is worse than crap, it's
darker than a manager's heart, and you just ain't feeling up to the task
of putting this thing back on the boat…. who you gonna call? ACL mode
1, that's who. I have the distinct pleasure of flying an airplane at
work that will couple to an ILS and land itself. When do I use it? Well,
as I write this in the hotel in Anchorage this evening, staring at a
five hour flight back home in the middle of the night, knowing that I'll
be pretty frkkin tired upon entering the approach phase; it's comforting
to know that if I'm feeling less than 100 percent, I can just "autoland"
the jet, pack my bags and head home in the morning rush hour traffic.
Plus, when the visibility is VERY LOW, (roughly 1000' or less) we conduct
what's know as a Category IIIb approach, and are NOT allowed to do a
manual landing. The aircraft will autoland every time in that weather.
We couple the machine to the ILS, and become VERY attentive watchers.
Of course, we have several checklists to do before we do this, and they
basically have us check that all the equipment is working correctly,
i.e., all three autopilots, etc. Any sign of a malfunction, (and there
are lot's of lights, bells and whistles to let us know), and we execute
an immediate missed approach. Jane's has (thankfully) modeled the Automatic
Carrier Landing mode here very well. It's a wonderful tool to use, turn
it on and let it do it's magic for you. I use it, and I'm not afraid
to admit it.
So, to sum up the A/P-flight guidance system is this effort, it ROCKS.
Experiment with it, use it, get comfortable with it, and it'll be there
for you when you most need it.
Well that's enough of the "jillion dollar, faster than the speed of heat,
everyone's a movie star" world of fighter jets; let's get down and dirty
in the world of combat helo aviation. I've been very fortunate to have
found myself in the cockpits of Razorworks new release, Comanche vs.
Hokum, since shortly after it's release. And I will say one-thing…WELL
DONE guys. I'm liking lots of things in this world, but let me just touch
on some of the autopilot functions that are new.
You're treated to the old "fly to a waypoint" version of the horizontal
axis of this autopilot, but they've spawned three new vertical modes
for us. One you use totally hands free, one you use with the collective,
and one you use with the cyclic…. bit of a smorgasbord, eh? They are
known as "Stable Hover Hold", "Hover Hold", and "Altitude Hold" respectively.
Don't get these mixed up, the first two are used in conjunction with
the autopilot, and the last one is used while you are "hand flying".
Let's break them down to see what they'll do for us, starting with "Altitude
Hold".
The manual states that the "altitude hold" will ATTEMPT to hold the machine
at the current radar altitude "in effect, terrain following for nap of
the earth flying". Don't bite off on this "terrain following" and "nap
of the earth" advertising. It will in fact keep you from "tying the low
altitude record", but you had better make sure the terrain resembles
Kansas and not somewhere in the mountains. How it plays out is that you
will engage it at whatever altitude, say 200' (you can increase or decrease
by 1 ft. [or 1 meter in the Hokum] with key commands) and the "hold"
feature will keep you at (or nearly at….especially over flat terrain)
that altitude, but you are still flying the helo BY HAND in relation
to the direction and speed. One big caveat, if you push forward on the
cyclic, the "ALT hold mode" will maintain your altitude, and you will
begin to pick up speed, but the only way to do this is by increasing
the torque of the engine. Warning, you will find that the machine will
over-torque itself to try and reach that speed, and if you do this enough
times, or long enough, the gearbox will fail. And again, this definitely
IS NOT a terrain following function; it will fly your ass into the hills
if you believe that it might be. Use it, but know its limitations.
"Hover hold" is basically a station keeping mechanism that can be engaged
when the speed of the machine is below a certain value (20 kts for the
RAH-66, 40 Km/h for the KA-52). This is THE moment in helo flying. You
are racing down a river bed, etc., come to a hover behind an obstacle,
do the bob-up dance, and send targets to the junkyard, is better than
bad sex….well, maybe. And the ability to hover in one spot is a mandatory
maneuver for any helo pilot. This mode will allow you to do all of that
with one very glaring problem. You will be applying pedal pressure to
keep the nose where you desire, but in this mode, you will also be responsible
for maintaining the ALTITUDE with your collective control. Normally,
not a big deal. But when you're looking through the KA-52 periscope,
or slaving the FLIR, you're be finding yourself becoming "unmasked" at
exactly the wrong times. Ever have the dream that you've gone to school
dressed only in your underwear? Well, that would be a blessing when compared
to doing this…you'll find yourself very dead, very quickly. So what's
the answer?
"Stable Hover Hold", that's the answer. You'll do all the same things
in terms of station keeping, but the computer chip buried deep in your
avionics bay will be adjusting your torque to maintain that altitude
for you. Much easier to use a full screen periscope or FLIR view to do
your business, when you're not having to use both hands, both feet and
God knows what other appendage to control the collective (Gene Simmons
of Kiss would most probably be the only one on the planet that could
do it…ugly picture, eh?). BTW, I engaged each of these modes at altitudes
above 10,000' MSL, so there doesn't seem to be a hard and fast altitude
restriction here. Why one would hover one of these helos that far OGE
(out of ground effect), is beyond me, but it can be done.
Anyway, that's about it for this little B.S session on A/P-flight guidance
systems. So what happened in the cockpit of that 757 you say? Well, in
a nutshell; the F/O was flying and they were approaching the airport
from the north. The planned approach path was to overfly the airport
and land from the south, and all the flight guidance systems were programmed
to do just that. The approach controllers asked if they wanted to land
"straight in" from the north….and they bit. They got rushed, the Capt
selected a nav point from the FMC (flight management computer) database
for the autopilot to fly to, and (unfortunately) of the two beacons with
that same identifier, the FMC thought they wanted the one on the OTHER
side of the mountain range several hundred miles away. He selected it,
the FMC told the autopilot "go there", and the A/P made a HARD LEFT TURN
heading that direction. As they were trying to "fix" this, the GPWS (ground
proximity warning system) began to blare, and they did exactly the right
thing…..they added power to do a missed approach, climb to a safe altitude
and figure it all out. But they crashed you say, WHY? Well, getting hurried
in any airplane is ugly, and when they found themselves high and fast
for a "straight-in" approach, they F/O did the right thing and extended
the speed brakes….they just didn't retract them on the missed approach.
Not even Mr. Boeing's wonderful 757 can fly out of a mountain range with
the "boards" sticking up on the wings. I guess the moral to that story
is 1) try not to get rushed in an airplane…ever, and 2) (as it pertains
to A/P-flight guidance systems) if the A/P does something really weird,
turn it off and "do some of that pilot shit Mav"….remember, ask them
to do something stupid, and they gladly will.
Good luck, good hunting, and I'll see you in the sky….
BBall
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