777’s autopilot. How it works? How to use various modes and how to figure out what all acronyms mean? Quick guide for Boeing 777’s newbies.
That’s correct – Boeing’s – not necessarily 777’s. The following description explains how most Boeing planes work – with the exception of 737 that uses older AP. The differences between 747s, 757s, 767s, 777s and 787 are small and in general – if you understand one of them you should have no problem getting to know any other.
Why am I describing the A/P?
I remember what I felt when I have bought my first “pro” 767 add-on. It was my first serious airliner and the first plane equipped with FMC. After years of FS-autopilot use in default planes I felt like I have hit a wall. It was totally different experience after my flights in Beech Baron, Kingair and Mooney Bravo. All I needed to do in those planes was to press HDG or NAV button, set altitude and the plane flew itself. In 767 I had to program FMC, learn LNAV and VNAV principles… I tried to search the internet for some tutorials but they seemed even more complicated than the manual.
As you can see from this introduction – I wrote this article for “airliner-newbies”. If you have already mastered the FMC and autopilot of an advanced add-on – I suppose you may skip this text. Or maybe not – unless you are a really hardcore virtual pilot – there may be something useful for you too.
Manuals are fun! (or RTFM!)
This is an introduction to the automated flight systems. I do not intend to replace the manual – AP part of PMDG 777’s manual is much longer than what I will write here. My intention is to give you a basic information. Hopefully – that will enable you to fly or learn faster.
When I think about my flight I always divide it into several levels of automation. The top level consists of LNAV and VNAV autopilot modes. Pitch and bank modes like HDG or TRK, FLCH, V/S, FPA and Altitude HOLD are below. Basic pitch and bank hold mode is even lower and manual steering is at the bottom.
What this scary acronyms mean?
LNAV – lateral navigation. Autopilot mode that makes your plane follow programmed route (waypoints, airways, etc).
VNAV – vertical navigation. Autopilot mode that controls height and climbs or descents. Following altitudes of your programmed flight profile.
HDG (HOLD / SEL) – heading hold / select.
TRK (HOLD / SEL) – track hold / select.
FLCH – flight level change.
V/S i FPA – vertical speed / flight path angle. In this mode the AP will climb or descend with the vertical speed you set (or keep the set flight path angle).
Altitude HOLD – pretty obvious…
Roll. Roll hold – do not look for a button with this description – I will describe roll mode later.
I did not mention LOC and APP modes earlier – both are used during approach.
LOC – localizer. Lateral mode that uses localizer signal (during ILS and LOC approaches).
APP – approach. Vertical mode that uses ILS glide path signal to descend (during ILS approaches).
A/T – auto throttle.
Roll is commanded by the following AP modes:
Pitch is commanded by these modes:
- Altitude Hold
Armed / Active
The difference between armed and active mode was another source of my uncertainty. What is the difference? When the armed mode activates?
When the mode is armed – it will activate automatically when certain conditions are met. Usually when the plane is in such position that the armed mode can take over.
Armed mode is indicated by white text on primary flight display. Active mode is indicated by green text.
An example – localizer mode
When the plane is being vectored to ILS approach pilot may use HDG select mode. To intercept localizer he should arm LOC mode. AP will change mode (activate LOC mode) as soon as the plane is in correct position to be guided by localizer signal.
How to use this modes and AP buttons?
AP – autopilot roll mode
The most basic (and rarely used) AP lateral mode is roll hold. If your Flight Director is turned off and AP button is pressed the plane will either hold heading (or track) or keep the current bank if it was rolled more than 5 degrees.
Heading vs Track
Heading is a direction in which the nose of your aircraft points.
Track is the line over the surface of the Earth that your planes flies.
In no wind situation heading and track are the same. Also – with a direct headwind and tailwind they will be the same. On the other hand – the stronger the crosswind component is – the bigger the difference between heading and track.
HDG HOLD (SEL) / TRK HOLD (SEL)
Heading or track hold or select.
HDG/TRK switch is used to choose between these two modes.
HOLD button will cause the plane to keep it’s heading (or track). Pressing SEL allows you to select the heading – the plane will turn to the heading dialed on the screen above.
The important difference between HOLD and SEL is in how the plane behaves when heading selector knob is being turned. In HOLD mode – it will hold your heading. In SEL mode – it will turn as you turn the knob.
In SEL mode you can decide on the bank angle limit – bank angle selector allows for 5, 10, 15, 20 and 25 degree limits or sets bank angle in automatic mode.
Lateral Navigation. As it’s name suggests – in this mode the plane will turn to keep assigned track. From the programmers point of view – it’s the most complicated lateral mode. I really enjoy the quality of PMDG 777 programming here – it’s better than some of the other autopilots in FS add-ons – you can admire this when arriving or departing using complex RNAV procedures.
For the pilot – LNAV is the easiest mode to use. As long as you have your FMC programmed – the plane will fly the assigned route. Actually I flown a few routes in LNAV mode from takeoff till landing – with no need of using other modes.
If you are away from the assigned route – you can arm LNAV mode and fly towards the magenta line on the screen using HDG SEL mode. If you are flying away from the assigned track when arming LNAV mode – FMC will worn you about that.
Vertical navigation. Similarly to the lateral – it’s fully automated mode that follows altitude profile programmed in FMC. Additionally – it takes over the autothrottle and controls your speed.
VNAV is activated at 400 ft over terrain (or more). Below it’s only armed (usually on the ground – before the take off).
When you are in VNAV – the AP will climb and descent according to altitudes programmed in FMC but you have to “allow” for the climb or descent by setting the altitude on MCP and pushing the selector button.
Altitude hold. Press it and the plane will hold it’s current altitude. Really simple.
It does not matter what altitude is dialed on MCP. Keep that in mind – it’s different from how AP works in most freeware or default FSX planes.
V/S and FPA
Vertical speed modes. The plane will keep selected speed or flight path angle.
FLCH – flight level change
Flight level change. As the name suggests – vertical mode allowing for changes in altitude (flight level).
In my first encounter with Boeings – it was the most mysterious mode. It certainly requires a careful study. Unlike the VNAV – it does not require any FMC inputs – you just select the altitude on MCP and it will take you to this altitude. When compared with V/S or FPA – it seems more… clever as it adjusts the climb speed to the altitude difference adding 500ft/min for every 1000ft of altitude change.
For me it one of the most useful modes for altitude changes.
It’s worth keeping in mind that if you change speed when changing altitude – FLCH will prioritize speed change. That means that it may level off in descent to decrease airspeed and then it will resume descent.
LOC and APP
Localizer and Approach. Quite obvious, isn’t it?
LOC – lateral mode used for flying localizer and ILS approaches.
APP – activates LOC as lateral mode and intercepts glide slope (G/S) of ILS approach.
VOR/LOC or NAV – there is no such mode here
You may have already known it but if you did not – there is no VOR/LOC or NAV mode in 777’s AP.
If you need to fly VOR radial – use LNAV or HDG/TRK SEL mode. I will describe how to do it in my next test.
Which vertical mode should you use?
All of them!
VNAV – I set it as a default mode for departure – it’s convenient if I use SID procedure.
FLCH – in all situations when I am cleared for an unrestricted climb – FLCH will take me to my altitude.
V/S – when I am cleared to an altitude over a specified point and I have no time to set it in FMC.
FLCH – during step climb or any other cruise altitude adjustments.
FLCH – cruise altitude adjustments.
V/S – when I want to get to a specified altitude at a specified point (different from what is programmed in FMC).
VNAV – when I have my descent profile programmed in FMC.