Misinterpretation of Aircraft Control Modes in the DSTG Book on MH370
Brian Anderson
14th December 2015
- Introduction
The Australian Transport Safety Bureau (ATSB) published an update to their report entitled “MH370 – Definition of Underwater Search Areas” on 3rd December 2015. At the same time, the Australian Defence Science and Technology Group (DSTG; formerly the DSTO) through the above ATSB webpage made available a book entitled “Bayesian Methods in the Search for MH370” (hereafter, DSTG book).
Some of the statements made by the DSTG authors warrant further attention and explanation. Specifically, the statements made in relation to control angles and headings in 6.3.1 thru 6.3.5 of the DSTG book are inaccurate and misleading, although the way that these factors are incorporated in the analysis may not change the result of the study.
In this post I explain how and why those statements are misleading, why the errors made might be of fundamental importance in the search for MH370, and indicate what could and should be done in terms of correcting the analysis. The intent here is simply to identify mistakes made in order to assist the official search teams.
- Lateral Control Functions
All the descriptions I have read of the roll functions for the B777 aircraft – and probably the Continental B777 Training Manual is the most detailed on the subject – indicate that magnetic parameters are calculated with reference to a look-up table of magnetic declination by using inputs of angles, velocities and position derived from the Inertial Navigation System (INS) and earth references (as opposed to the magnetic parameters being directly measured). For example:
Ref: section 34-20-00, page 41 of the Continental Manual (page 1839 of 3328)
Magnetic Heading And Magnetic Track
Rotational rates and linear accelerations go to the rotate accelerations to earth reference function. The rotate accelerations to earth reference function calculates accelerations in relation to the earth. The accelerations in relation to the earth go to the velocities integrator. The velocities integrator calculates velocities. The velocities go to the position integrator and the calculate magnetic parameters function. The position integrator calculates airplane position.
The calculations described above are performed in the ADIRS – ADIRU system, shown in block diagram form in section 34-20-00, page 43 of the Continental Manual (page 1841 of 3328), as below: 
.
The roll control functions are described as follows:
Heading/Track Hold
In this mode, the airplane holds either heading (HDG) or track (TRK).
If the HDG/TRK display on the MCP shows TRK, the airplane holds track.
If the HDG/TRK display on the MCP shows HDG, the airplane holds heading.
Heading/Track Select
In this mode, the airplane turns to the heading or track that shows in the heading/track window.
If the HDG/TRK display shows HDG, the airplane goes to and holds the heading that shows in the heading/track window.
If the HDG/TRK display shows TRK, the airplane goes to and holds the track that shows in the heading/track window.
It is not explicitly stated in the manual whether the aircraft will (at least over a period of some hours) hold the heading or the track if the heading display is in Norm (i.e. displaying a magnetic heading, and thus follow a curved path).
(With regard to the distinction between the terms “Heading” and “Track”, please see the addendum at the end of this post.)
Further evidence that magnetic angles are in fact calculated and not measured can be ascertained from Aero Quarterly (issue: Qtr 04/09), as published by Boeing, discussing Correcting the Effects of Magnetic Variation, and emphasizing the importance of airlines updating their inertial reference systems to the latest magnetic variation (MagVar) tables in order to avoid potentially hazardous magnetic heading-related navigation errors.
The Heading- and Track-Select functions serve only to turn the aircraft onto a new Heading. Once that Heading has been acquired the roll control functions serve to maintain the Heading or the Track automatically.
Considering the possibility that the Heading or Track may be displayed with reference to True North [True], or Magnetic North [Norm], there are therefore four possible combinations of Heading and Track Hold that might ultimately determine the path of the aircraft, as follows:
(i) TRK Hold [True]
In this mode the track of the aircraft is maintained constant with respect to True North, and small instantaneous adjustments to the aircraft heading are performed to compensate for wind vectors so as to achieve this. The resulting flight path describes a loxodrome.
(ii) HDG Hold [True]
In this mode the heading of the aircraft is maintained constant with respect to True North. No adjustments to the aircraft heading are performed to compensate for wind vectors. The resulting flight path would describe a loxodrome in zero wind conditions, but is otherwise continuously modified by wind vectors.
(iii) TRK Hold [Norm]
In this mode the track of the aircraft is maintained constant with respect Magnetic North, and small instantaneous adjustments to the aircraft heading are performed to compensate for wind vectors to achieve this. The resulting path will be curved. Since the magnetic heading is calculated in the ADIRS – ADIRU system the aircraft heading (with respect to the earth) must be continually modified and updated by incorporating the declination obtained from the MagVar tables relating to the current aircraft position, and the wind vector.
(iv) HDG Hold [Norm]
In this mode the heading of the aircraft is maintained constant with respect to Magnetic North. No adjustments to the aircraft heading are performed to compensate for wind vectors. The resulting path will be curved as in (iii) above in zero wind conditions, and is otherwise continually modified by wind vectors. Again, the aircraft heading (with respect to the earth) must be continually modified and updated by incorporating the declination obtained from the MagVar tables relating to the current aircraft position.
- Lateral Navigation
Lateral Navigation (LNAV) is a specific control mode whereby the aircraft typically follows a series of waypoints previously stored as a flight plan in the Flight Management System (FMS). The track followed by the aircraft between waypoints is (a segment of) a Great Circle. The aircraft heading is continuously adjusted for wind vectors to maintain this track.
Waypoints can be defined in many ways. They are not limited to predefined Instrument Flight Rules (IFR) waypoints, and can be entered into the FMS (more specifically, the Control Display Unit, or CDU) to modify the flight plan at any time. For example it is possible to describe a waypoint simply as a set of latitude and longitude coordinates (such as “N071.2E0953.15”, which represents a waypoint at N7° 1.2’ E95° 3.15’, and is displayed as N07E095).
Section 6.3.5 of the DSTG book states, in reference to the LNAV control mode:
“Expert advice indicates that if the autopilot system is operating in lateral navigation mode and it reaches the final programmed waypoint, then it reverts to the previously selected heading hold mode.”
This statement is at variance with the description of LNAV operation in the Honeywell B777 FMS Manual, and with the description found in a variety of other B777 FMS references.
The Honeywell description of the LNAV action after overflying the last programmed waypoint is this:
“. . . DISCONTINUITY is displayed in the scratchpad, and the aircraft maintains its existing track.”
Other references state:
“LNAV maintains current heading when passing the last active route waypoint.”
The different terminology used in different references (Track vs Heading) may simply be one of common usage of the terms when describing an instantaneous point in flight (i.e. the instant that the discontinuity occurs).
It is interesting to note, from the Honeywell manual, that:
“Only the active waypoint course can be referenced to magnetic north because the ADIRU can provide magnetic variation only for present position.”
This would suggest that the course followed after passing the last waypoint, which by definition cannot be an active waypoint course, cannot therefore be referenced to magnetic north.
- How is this relevant
Considering the aircraft path southwards from a position in the northwest of the Malacca Strait:
(i) If this path was being managed in LNAV mode (with or without manual input), then the final turn south must have been at or about a predetermined waypoint, and furthermore the track from that point must be a Great Circle to another waypoint that had previously been manually entered into the FMS via the CDU. Clearly, manual input of at least two waypoints is therefore necessary at some time before reaching the final turn in the Malacca Strait. After overflying that final waypoint (if indeed it was actually reached), then the aircraft would be expected to continue on the same track. At that point the track may become a loxodrome.
(ii) There is no evidence to exclude the possibility of some flight manoeuvres towards the end of the track in the Malacca Strait, which would have delayed the commencement of the flight south. For example it is entirely possible that the aircraft completed an orbit of greater than 360 degrees, perhaps 460 degrees even, before proceeding south. Any delay or manoeuvres such as this can be shown to still fit the available satellite data, in particular an intercept with an arc meeting the BTO at 19:41 UTC. However it seems most unlikely that such manoeuvres would have been manually entered in to the CDU to become an active flight plan.
(iii) An alternative to the final turn south being a turn at a waypoint is the possibility that the turn was initiated manually using the Heading- or Track-Select controls. After selection the aircraft rolls onto the desired heading or track, and then holds the selection, as described in 2(iii) and 2(iv) above. (Note that this selection also effectively overrides the LNAV function.) It is therefore necessary to consider flight paths consistent with these control modes.
(iv) We do not know the control mode in use at the time of the final turn south. However, one might be biased toward Norm (magnetic reference) rather than True, since Norm is the de facto standard for most flight manoeuvres where these control modes would be used. For example, virtually all tracks and headings described on aviation navigation charts are referenced to Magnetic North, as are virtually all instructions from Air Traffic Control (ATC).
(v) While TRK Hold [True] would result in a relatively straight path (a loxodrome) from the final turn, each of the other three control modes would result in curved paths, in fact curving significantly eastwards as the flight progressed, due to increasing westerly winds at more southerly latitudes, and the magnetic declination increasing more rapidly.
(vi) The DSTG analysis seems heavily biased toward straight paths after the final turn south, and it is therefore not surprising that a “hot spot” at about 38 degrees south results. On the basis that it seems equally likely that curved paths may have been followed, or indeed the turn south commenced further to the NW in the Malacca Strait, or occurred at a later time, it is of concern that termination points further NE on the 7th arc have not been shown to be viable, and perhaps even equally probable, given our lack of knowledge of the aircraft control modes in operation during MH370’s flight southwards.
(vii) It can be shown that curved paths, such as might result from control modes described in 4 (ii), (iii) and (iv), can also fit the Inmarsat-derived data, and end on the 7th arc, but further to the northeast on that arc than the “hot spot” indicated in the DSTG book and on the cover of the latest ATSB report update. Such curved paths, however, are not at constant speed. Speed variations, and in particular significant slowing of the aircraft towards the end of the flight, are necessary in order to fit the Inmarsat data with these paths.
(viii) While TRK/HDG Hold [Norm] have their place in managing aircraft navigation – and in particular in respect of standard procedures, navigation chart information and under instruction from ATC – the requirement in these situations is for only relatively short times and relatively short distances. In such situations the magnetic declination does not change significantly and the resulting aircraft azimuth remains essentially fixed. Furthermore, that azimuth would be precisely the same whether the displayed track or heading was referenced to Magnetic or True North. In contrast, maintaining TRK/HDG Hold [Norm] over lengthy times or long distances has absolutely no value. The fact that either of these two modes may have become the default lateral control mode over something like the last six hours of the flight of MH370 serves only to compound the problem of identifying the final resting place.
- Conclusion
The analysis in the DSTG book should be reviewed after correcting the misinterpretation of the aircraft control modes as presented in Chapter 6 of that book.
Addendum at the suggestion of Barry Martin:
Many readers will be confused by what “Heading” means as opposed to “Track”. The diagram below shows Heading Hold at the top, and Track Hold beneath it. Source: here.
