I finally added the last panel for the autopilot of the Lockheed L-1011. The panel that completed my set is the HEADING/PITCH MODE panel. It's actually a really neat device, it has a vertical speed dial that is synchronized to the vertical speed of the aircraft and can also be overridden with the dial to the right of it. The idea is that in control wheel steering the pilot would pitch the aircraft to the correct vertical speed and then engage the autopilot. The panel also has the heading input for the automatic flight system.
Below is a picture of the complete autopilot of the Lockheed L-1011. This one is completely analog, the -500 models also had a digital version.
The other switch elements on the panel are:
- VS ... Vertical Speed (Hold)
- ALT ... Altitude (Arm / Hold)
- IAS ... Indicated Airspeed (Arm / Hold)
- MACH ... Mach Speed (Arm/Hold)
- HDG ... Heading (Arm/Hold0
The various modes will be indicated on the AFCS Mode Indicators.
Panel Location
Do the IAS and Mach modes use the autothrottle to maintain speed, or the aircraft's pitch attitude?
ReplyDeleteGreat question George. On the L-1011 the Indicated Airspeed (IAS) Hold and the Mach Hold are "pitch" modes. For IAS and Mach Speed variations are sent to the L-1011s pitch computer from the hold mode which results in a speed/pitch error being sent to the ADI and that then results in a an action by the pitch servos.
ReplyDeleteThe Auto Throttle System (ATS) provides thrust control to either a selected airspeed or to airspeed commands coming from the thrust management or flight management system. When the ATS is engaged, speed is controlled solely through the throttles and all other pitch speed modes (IAS or MACH) are disengaged. The opposite is also true, if the aircraft was maintaining speed via the ATS and a speed pitch mode is engaged (IAS or MACH) the ATS will be disengaged.
More questions on the original version of the autopilot:
ReplyDelete1. Is the ATS switch spring-loaded, springing back to the OFF position if a speed pitch mode is engaged?
2. Do the IAS pitch mode and the ATS both use IAS setting under the ATS switch?
3. How does the pilot adjust the desired Mach number when using Mach pitch mode?
Thanks for your help...
George, thanks for the comment.
ReplyDelete1. The ATS switch is solenoid held (spring-loaded) in the "Engage" (top) position. In other words if the ATS system is active (at least one ATS channel checks out good) the solenoid activates and allows the switch to be moved from the down to the top position ... where the switch is held until either moved back down by hand or ATS is disengaged by another mode engagement or ATS system failure.
2. No, the IAS setting for the ATS and the IAS and Mach pitch mode have no direct relationship. Actually, the ATS and the Pitch Mode system of the L-1011 auto flight system are completely independent. The IAS pitch mode uses the current speed as displayed on the Airspeed indicator. Basically, the auto flight system will hold the airspeed at the moment of IAS mode engagement. The pilot is expected to fly the aircraft to the correct speed and then engage the pitch hold mode.
3. The pilot would adjust the Mach speed by flying the aircraft to the correct Mach number as indicated on the Airspeed indicator and re-engaging the Mach hold mode.
Also, under what conditions does the VS indicator move as the aircraft's vertical speed changes, and under what conditions is it free for the pilot to select a vertical speed? Does the pilot need to select VS mode first and then move the vertical speed to the desired value?
ReplyDeleteThe small square vertical speed indicator is constantly updated from the actual vertical speed of the aircraft. So, let's say the pilot pitches the aircraft up to 1000 fpm by hand then the VERT SPEED would now also indicate 1000 in the blue range of the indicator. If the pilot now presses the "VS" hold switch (and the autopilot is in the right state) the aircraft will hold the 1000 fpm nose up pitch attitude. The VERT SPEED, once engaged, can be adjusted to the desired value with the UP/DN thumb wheel. So if the vertical speed should be 800fpm vs. the 1000fpm the aircraft was flying while the VS hold was engaged, the pilot could push the thumb wheel up to 800 in the blue range which will effectively pitch the nose down and cause the aircraft to climb at only 800 fpm.
ReplyDeleteIn which other modes though (other than VERT SPEED) is the pilot free to change the vertical speed setting using the UP/DN thumb wheel?
ReplyDeleteAnd also (though a bit off topic) I saw no "INS" button on the original autopilot panel you photographed -- how does the pilot do INS navigation?
(If you're wondering why I'm asking all these questions -- I'm responsible for this set of Microsoft Flight Simulator panels, and I was thinking about making the autopilot more realistic...)
The thumb wheel next to the VERT SPEED windows only works when the VS hold mode is engaged. Other than that, the wheel has no other function and turning it does not result into anything.
ReplyDeleteYes, the "INS" navigation mode is a bit confusing in the pictures. The analog APFDS panel did not have that, it was introduced as part of the digital autopilot (which is what the drawing is from ... i should really fix that and use a drawing for the analog autopilot). The analog APFDS used "NAV" as the mode that used the VHF radios sources for navigation. There are actually several renditions of the NAV MODE panel depending of what the capability of the NAV system in the L-1011 was. The one in the photograph is the most basic one that uses VOR capture and track for NAV mode. In this case the spring loaded rotary switch only says "SPLIT", "1" and "2" for the VOR source used. The more updated version of the same panel has a rotary switch with positions ""VOR", "FMS" and "OMS". In this case the NAV source as the navigation reference is selected by the rotary switch. The INS switch on the digital autopilot panel simply replaced the rotary switch with selecting between "VOR" based NAV or "INS" (FMS/OMS) based NAV.
So you're saying that in any mode other than VERT SPEED, the vertical speed indication is locked to the actual vertical speed of the aircraft then, so there is no ability to select a vertical speed value in advance?
ReplyDeleteThat is correct. Vertical Speed is not selected in advance on the L-1011 APFDS. The VERT SPEED indication and existing vertical speeds are coupled together when VS is not engaged and changes as VS speed changes. Once VS is engaged, the vertical speed control wheel can be used to make adjustments.
ReplyDeleteWhat's the different between the "ALT" button on this panel, and the "ARM" button above the altitude window on the extreme right of this post's second image?
ReplyDeleteAnd could you explain "Arm" versus "Hold" for the IAS, MACH and HDG mode please? Sorry to be so annoying...
George, no problem. Not annoying at all ... I love this stuff. So here it the explanation for the ALT as hold feature and the ARM for Altitude Select.
ReplyDeleteThe "Altitude Select Arming Switch" when pressed the first time arms the APFDS for automatic capture of the selected altitude. It itself does not cause any pitch change of the instrument. It simply tells the APFDS computers to start the level off program if the altitude comes within what I think about 1000ft of the selected altitude (have to reference the book for that one). The ARM switch light remains illuminated until the selected altitude is reached. ALT ARM also appears on the AFCS Mode annunciators. Once the altitude has been reached the ARM light goes out and the ALT HOLD mode is automatically engaged and the ALT light on the mode side of the autopilot illuminates. The selected altitude has now been captured. If the still illumined ARM switch is pressed for a 2nd time, the ARM procedure is canceled and altitude that shows in the select window will not be captured.
As far as I know there is only a "Arm" mode for Altitude ... all the other pitch modes (IAS, MACH and HDG) will simply hold the respective value at the moment the switch is pressed. The HDG hold mode has some interesting features as well:
HDG provides heading reference for the flight director and/or AP in CMD mode. HDG can not be engaged if any NAV or APR mode is active and captured. HDG mode can also not be actives in TURB (Turbulence) mode. The Heading Control Knob underneath the HDG hold mode provides control to APFDS heading reference system. The Heading is also repeated on the HSIs. This is how you set the heading on the HSI when the aircraft is not in NAV or APR mode.
OK, so let's see if I understand this correctly:
ReplyDelete1. Pressing "ALT" engages ALT HOLD mode, which holds the current altitude.
2. Pressing "VS" (for example) will maintain the selected vertical speed indefinitely if "ARM" mode is off. If "ARM" mode is on, the autopilot levels out at the indicated target altitude and then switches to ALT HOLD mode.
My question about HDG is that I know that on the Boeing 757/767, one can either push the HDG HOLD button (which holds the aircraft's current heading) or push the HDG selection knob itself, illuminating a green light on that knob and turning the aircraft towards the selected heading. Once the selected heading is attained, the green light goes off and the HDG HOLD light comes one. To change heading again, it is necessary to press the HDG selection knob again after selecting the new target heading.
Does the L-1011 have two modes like this (hold current heading, versus turn to selected heading)?
Thanks for your help...
Yes, there are quite significant different in the auto flight concept between Boeing and Lockheed. The B767/B757 had been built right from the start around the concept of a Flight Management System, while on the L1011 the idea of a fully computerized Flight Management System did not come until way into the life of the product.
ReplyDeleteThe L-1011 does not have a heading arm switch. So the heading select knob can not be pressed. Once Heading hold is selected either the current heading is held or the aircraft will send roll information to the Flight Director if the heading is changed. In Command Wheel Steering (CWS) mode the FD will simply show the direction to bank but nothing will happen. If the AP is engaged in Command (CMD) mode then the aircraft will actually execute the heading change.
So the idea would be that you point the aircraft to the heading you want, then dial the current heading into the select window and then engage the heading hold mode. Or, dial in the heading you want to roll to and then engage the HOLD mode with the AP in CMD mode and the aircraft will head for it.
By the way ... this might be of interest in your program. The L-1011 only allows a maximum bank angle of 28º at 200 knots, decreasing to 15º at 450 knots.
450 knots - wouldn't that be in excess of Vmo? Or is that 450 knots TAS rather than 450 knots CAS?
ReplyDeleteThat's correct. TAS.
ReplyDeleteIs there a minimum Mach number to engage MACH hold (with selection at a lower Mach number turning on IAS mode instead)?
ReplyDeleteThe minimum engagement speed is 1.3 x Vs.
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