Brittain Main. Positive Control System. Course and Heading. Altitude Control. Tidbits and Rules of Thumb. Parts Gallery.
Altitude Hold
By far, this is my favorite portion of the three-axis autopilot system
This system uses zero electronics to maintain pitch and altitude.
Theory
My understating of this system is that the ASI and a “dynertial box” work together to maintain a constant pitch in flight. The pitch can be level, climbing, or descending. Pitch control is initiated by pulling the “Pull PITCH On” button (which is usually mounted far away from the B-6 panel - near the pilot’s right knee).
The pitch system is then “built upon” (kind of like how the course and heading builds on the working PC system) with an “altitude reference chamber” and an analogue computer. to maintain altitude.
When the ALT HOLD is pulled on, it shuts a valve to the altitude reference chamber. This trapped reference pressure is then used by the altitude “box” as a comparison to direct the pitch chamber to pitch up or down to maintain the correctly matching pressure.
Operation
Typical use of the system:
Climb out normally, when at a safe and comfortable altitude and when at the climb rate that the pilot would like to hold, pull the “Pitch Trim” button. The pitch system will now attempt to hold the climb rate balancing IAS from the pitot tube and through a weight in the dynertial (to counter act gusts).
When at altitude, trim forward to a level altitude. Allow speed to increase to cruise speed (alternatively, climb to 40’ above altitude, trim to level, then push forward to altitude until 20’ above altitude). Then pull the ALT HOLD button (on the B-6") to maintain altitude.
When ready to descend, push the ALT HOLD button back in, retrim and set power for descent (all the while keeping the pitch button ON/pulled out). When nearing the pattern or the pilot’s normal hand-flying location, push the PITCH button back in.
An update from Vance Harral:
Regarding the pitch hold system, suggest you avoid using the phrase "hold a climb rate". I know the Brittain manuals use this phrase too, but the system is not connected to the VSI or any other instrumentation that provides rate-of-climb information. Rather, the system is actually trying to hold a specific combination of airspeed (it has a pitot input), and "G" load (this is the inertial weight I posted about). It's true that over a small period of time with little perturbation, holding a constant airspeed and 1G load factor will result in a relatively constant climb/descent rate. But the engineer in me thinks that stating that the pitch hold system tries to "hold a climb rate" is misleading.
Operational Limits
The airplane will hold this altitude for a long duration. When the altimeter setting changes, the reference chamber will still be using the older barometric pressure and will need to be temporary disengaged (ALT HOLD pushed in) and then reengaged to capture the local pressure at the correct altitude (ALT HOLD pulled out).
Reference and Useful PDFs
Mooney Pitch Control Operation & Services instructions Manual No. 11968-2 (Automated 3/31/01)
Photos
Altitude Hold System (mounted in tail section)
This photo shows the pitch chamber (gray hoses), the Altitude chamber (under the pitch chamber), and altitude reference chamber (block of aluminum).
Pitch Dynertial
The insides of a the pitch dynertial.
Useful comments from Mooneyspace.com
N601RX noted about the system (and the many variations):
That plugged line goes to the pitch indicator or to the on/off valve. I can't remember, but you should be able to find the install manual on here somewhere and it is easy to read. I am pretty sure it goes to the pitch indicator. Also, the entire control box should be installed 2" behind the firewall. The only thing in the tail should be the servos and altitude reference chamber. (The aluminum box at the bottom of the photo... It should actually be on the belly skin.) At the minimum, check your w&b and replace the cracked black hose going to the filter. That install doesn't look legal to me.
Vance Harral wrote up a very good analysis of his system:
To clarify, the "eyeball" indicator has a fixed side and a moving side, both with horizontal white lines as you say. The vacuum lines actuate the moving side, which is essentially an indicator of how much force the system is applying to the elevator. In other words, the vacuum lines are an input to the indicator, driven by an output from the altitude hold unit in your tailcone. The (negative) pressure in the indicator lines is a down-regulated analog of the (negative) pressure in the lines to the elevator boots.
The primary purpose of the eyeball indicator is to give the pilot information on what to do with the elevator trim when the system is engaged. If the indicator is consistently indicating an application of up elevator, the pilot should add some up trim, and vice versa. The goal is for the lines to match, most of the time. This indicates the most efficient trim setting, with the autopilot applying little to no elevator force in either direction. But note that when the autopilot is working, the indicator is constantly in motion to small degrees. You only adjust the trim if the average value is consistently above/below the fixed line.
In our system (B-5 instead of B-6 but the idea is the same), I find the eyeball indicator to have mixed utility at best. First, I always trim the airplane for level flight prior to engaging altitude hold, so the indicator is mostly neutral most of the time anyway. Momentary changes in the indicator aren't very interesting - again, that's just a confirmation the pitch actuator is actually working. If the indicator is consistently high or low for an extended period of time, that means you've entered a very extended up/down draft, or that one of the boots is starting to leak. A trim adjustment is appropriate in those cases, but those are pretty rare events. More importantly, though, I don't think the indicator is intuitive at all. Imagine for a moment that you look at it, and the movable line is higher than the fixed line. What exactly does that indicate? Should you trim up or trim down to fix it? There's a correct answer, of course, but it's not obvious.
My motivation to be really educated about the pitch trim indicator and other operational aspects of the altitude hold is hampered by the performance of our system to date. Our altitude hold "works", but performance is weak. We think this is a combination of two issues in our system. The first is the integrity of the seals for the tubing and valves associated with the altitude reference chamber. Ideally these would be perfect. In practice, they have a nonzero leak rate spec'd in the maintenance manual. What this means is that if your altitude varies from the selected altitude for more than a certain amount of time, the reference chamber establishes a new normal, and holds that new altitude instead of the one you originally selected. To prevent this, the system must be able to apply enough force to the elevator to correct altitude variances fairly quickly. That's our second issue. When we encounter an up/down draft, the system applies appropriate correction, but it's often too "soft". Before the system can return the airplane to the originally selected altitude, the reference chamber leaks enough to establish a new reference. We then have to disengage the pitch control, manually re-established the desired altitude, and re-engage. In smooth air this might happen once every 10-15 minutes, which really isn't too bad. In rough air - when you really want the altitude hold - it happens every couple of minutes. So often that it's arguably like not having altitude hold in the first place. We're hopeful performance can be improved by improving the reference chamber seals and/or increasing the gain on the elevator servos, but we haven't gotten around to addressing it yet.
My altitude hold works reasonably well. It might vary 50-75 ft per hr. It's made up of 2 parts, pitch control and altitude hold. The altitude control reference chamber and associated plumbing valve shouldn't leak any. It consist of a diaphragm with one side connected to the reference chamber and the other side exposed to cabin attitude pressure. The diaphragm is attached to a small valve that directs more or less vac to the up or down servo. If the altitude control chamber pressure matches the cabin pressure the diaphragm and valve are centered and direct equal pressure to both the up and down servo. It isn't hooked to the static port and is sensitive to internal cabin pressure/temp. Opening the vent window in leve flight will result in a almost full down deflection of the yoke.
The pitch control is connected to the airspeed pitot tube and has a calibrated leak in it. It uses airspeed as well as a pendulum to detect pitch changes.
When both are turned on the altitude hold and pitch control are plumbed in parallel with your servos and indicator.