The recent increase in GNSS jamming and spoofing brought to light a key issue amongst flight crews: A lack of detailed information about how the Terrain Awareness and Warning System (TAWS) is integrated on their respective aircraft. As we shall see, detailed knowledge of the avionics architecture can profoundly improve the crew’s ability to cope with abnormal situations.
Terminology: GPWS vs. EGPWS vs. TAWS
It turns out, pilots seem to be very familiar with the term EGPWS, even though the regulatory requirements refer to TAWS [1] [2]. Why is that?
After a series of Controlled-Flight-Into-Terrain (CFIT) accidents during the 1970’s, a global effort was undertaken to reduce these, by mandating a basic system called Ground Proximity Warning System (GPWS). This system typically used data from downward-looking radar antennas, barometric altitude, ILS glideslope deviation as well as gear and flaps position. It provided the so-called “basic GPWS modes” [3].
As this system proved inadequate for more demanding terrain situations and technology had advanced to provide forward-looking capability using a terrain database, a new system was introduced during the 1990’s. One leading manufacturer created the trade name “Enhanced GPWS” (EGPWS), and after being picked up briefly by the FAA during the 1990’s, essentially all authorities reverted to the more general term “TAWS” shortly thereafter. It is this term that is used in today’s official regulations.
There are many manufacturers of TAWS systems on the market, selling systems by the trade names of EGPWS, T3CAS, HTAWS, TAWS etc.
TAWS classes
Today’s TAWS systems come in different classes: A, B and C. A further class D is under discussion within RTCA [4]. The European regulatory framework mandates the more sophisticated class A TAWS for turbine-powered aircraft above 5700 kg MTOM or a MOPSC of more than nine, while reciprocating-engine-powered aeroplanes heavier than 5700 kg or an MOPSC of more than nine shall be equipped with a class B TAWS [1].
Similar regulations exist in the US.
For helicopters, the term Helicopter-TAWS (HTAWS) is often used, referring to a TAWS intended for helicopter operations. In Europe, these are mandated for helicopters in commercial air transport if heavier than 3175 kg or with a MOPSC of more than nine and initial CofA after 2018 [1].
The main characteristics of the different TAWS classes are depicted below [5] [6]:
Comments [5]:
Class A TAWS requires a display (MFD, EFIS or another compatible indicator).
Class A TAWS allows use of GPS, RNAV, IRS and other position sources.
Class A TAWS requires a 2500 ft radio altimeter to function as a backup to GPS and barometric altitude.
A class A TAWS unit can be installed to meet or exceed class B requirements — adding a significant level of safety.
Comments [5]:
Class B TAWS does not require a display. If a display is installed, it should meet the guidelines of Class A TAWS.
Class B TAWS must have an approved GPS for horizontal position. TSO-C129a, TSO-C145 or TSO-C146.
Class B TAWS does not require a radio altimeter.
Class C TAWS
The FAA recently introduced the class C TAWS, which is aimed at general aviation aircraft that are not required to be equipped with a TAWS. It contains basically the same requirements as class B, with some adaptation for general aviation, specifically: reduced warning thresholds [6].
Soon a class D TAWS?
There are ongoing discussions within RTCA to introduce a further class of TAWS (class D) to accommodate low-level VFR operations in demanding terrain. This is of particular interest in Alaska. Proposed modifications include the option to inhibit alerts, while keeping the terrain display available or time-limited inhibition [4].
Integration on transport category aircraft
It will be clear from the description above that transport category aircraft have to be equipped with a class A TAWS. Here is where it gets interesting: There are several avionics architectures found on today’s fleet, and it is important to know yours! The following paragraph describes three typical variants, based on [7].
Legacy
Historically there was no GNSS position available, and the TAWS was simply using the FMS position to compute the forward-looking alerts. It turned out, that created some potential for disaster: See Addis Abeba incident below.
Hybrid
By far the most common version in today’s transport aircraft is a hybrid integration where the TAWS receives a form of GNSS-IRS position. This is typically independent from the FMS. Therefore:
Deselecting GPS on the FMS will not affect the TAWS position source
The crew cannot typically “choose” the position source for the TAWS
A spoofed GPS location can potentially create nuisance TAWS alerts
When the primary TAWS position source is not available, it may revert to the FMS position in some cases
Stand-alone
In this layout, the TAWS receives the GNSS position directly from the receiver, without further augmentation by other aircraft systems. Like the hybrid version, the FMS position may be used in some cases, where the GNSS position is not available.
Implications on terrain display
The implications of the hybrid and stand-alone architecture are important:
The terrain display works independently of the FMS
The ND moving map and terrain data can “shift” independently of each other (spoofing case)
An example of a navigation display showing terrain data after failure of both FMGC’s on a A320 without “backup NAV mode” is provided below:
There is no FMGC position available, therefore no map. The terrain data is still displayed. The fact that the terrain only covers half the display in rose mode is normal system behavior.
Beware: Envelope modulation or mode 2 suppression
A very popular misconception is that the “GPWS basic modes” are not affected by de-selecting the forward-looking alerts. In fact, they can be affected, but not in the way one might think. Quite a few TAWS manufacturers use a feature called “envelope modulation” or “mode 2 suppression” to reduce nuisance mode 2 alerts [8] [9]. Remember, mode 2 is simply “excessive closure rate to terrain”. This is measured by a downward-looking radar altimeter. In a scenario where there is sharply rising terrain below the approach path, mode 2 may trigger even though the aircraft is following its intended path. This problem is typically overcome by suppressing mode 2 when forward-looking capability is available [8] [9]. And here is the culprit: If a flight crew elects to de-select the forward-looking feature, they might encounter a nuisance mode 2 alert, while being on the correct path. Definitively something to be aware of.
Incident example: Addis Abeba, based on [10]
This incident demonstrates in a mind-blowing way, what can happen if TAWS uses the FMS position, and the aircraft is not equipped with a GPS receiver. This represents the legacy architecture that was abandoned for good reasons.
On 31st March 2003, an A320 was on approach to Addis Abeba airport in IMC. The current AIRAC cycle contained only NDB and VOR approach charts for runway 25L, as there was an ILS approach just being installed for runway 25R.
As the aircraft was not equipped with a GPS receiver, the FMGC position was updated using VOR/DME information from the ADS VOR/DME. The crew were using VOR rose mode (PF) and ND map mode (PM), while the FMGC displayed “nav accuracy high”. Based on the VOR raw data and the information on the ND map, they were on the correct approach path. The crew noted that the NDB AB, which they had also tuned, showed them to the right of course. They attributed this to thunderstorms in vicinity. During the later stages of the approach, the VOR needle disappeared from view and the crew carried out a missed approach.
After a discussion with ATC regarding the serviceability of ADS VOR, the crew carried out a second approach. This time, the following occurred: While being indicated on the correct approach path, the crew were alerted by an untimely “400” callout from the radar altimeter. Shortly thereafter, the crew initiated a missed approach, during which a “too low terrain” TCF alert was triggered.
After that, the aircraft diverted to Djibouti. What had happened?
The crew were presented with a misleading VOR raw data indication, which was also used by the FMGC to update the FMGC position and furthermore by the TAWS to provide terrain warnings.
This led to the following approach trajectories:
On the second approach, the aircraft came within 56 ft of the terrain, while in IMC.
It turned out the ADS VOR had undergone some maintenance in the days prior to the incident and incorrect procedures had been applied when sealing the antenna. This allowed water to ingress into the antenna assembly and created significant bearing errors as indicated below:
In accordance with ICAO Annex 10, a monitoring system must be installed and remove the VOR signal, if the bearing error exceeds 1°. However,
The cables of the monitoring circuit had been severed during construction work, rendering the monitoring circuit inoperative.
Furthermore, no flight-inspection was carried out after the maintenance action on the VOR. This led to the unique circumstances where a single misleading navaid (ADS VOR) caused the raw data, the ND map and the TAWS system to display hazardously misleading information.
The crew were presented with seemingly correct raw data and ND map position (left), whereas in reality the aircraft was significantly right of course (right picture). With the correct indication on the right, there would have been timely terrain warnings.
Of note is also, that only the TCF alert “too low terrain” was triggered on the second approach, no other TAWS alerts were issued. The TAWS manufacturer also confirmed that TAWS envelope modulation (for mode 2) had been applied.
While this scenario is unlikely to be encountered with a more modern avionics suite, it demonstrates how critical it is for the flight crew to detect and react to even the slightest inconsistencies.
Accident example: Database coverage, based on [11]
In 2012, after a bizarre misunderstanding between the flight crew and ATC, a Norwegian C-130J Super Hercules aircraft descended below safe altitude while under IFR in IMC. The aircraft was enroute from Norway to Sweden, intending to land at Kiruna.
While at FL130, in IMC and 50 NM away from the airport, the crew advised ATC that they would later perform a visual approach. When the crew established radio communication with Kiruna tower, the controller assumed that the aircraft was indeed VFR and VMC and cleared them down to FL70. A clearance that took the aircraft out of controlled airspace and below the minimum usable flight level, which was FL100. The flight crew never cancelled IFR but failed to ensure terrain clearance. There was no radar coverage in the area.
So, what about TAWS?
Indeed, the aircraft was equipped with state-of-the-art TAWS and a military Ground Collision Avoidance System (GCAS). The flight crew selected “tactical mode” for these systems, which had dramatic implications: The global TAWS database was replaced by a proprietary Norwegian database, which contained no terrain data north of 60° N (where the accident took place). Further, the thresholds for the GCAS were significantly reduced. Tactical mode was never intended to be used in IMC, a limitation the crew were obviously not aware of. Flying north of 60° N, this placed the crew basically in a situation without terrain warnings (see below).
As evident from the CVR, the crew were blissfully unaware of the aircraft’s proximity to terrain and accepted the clearance down to FL70 without any discussion. The aircraft struck the ridge near the top of Mt. Kebnekaise while flying in IMC. The investigators were able to demonstrate that the selection of “TAWS normal mode” would have provided the crew with ample warnings.
This accident serves as a stark reminder of the basic responsibility of the flight crew to maintain terrain separation when flying under IFR. When being radar vectored or cleared “direct-to”, the controller becomes responsible for terrain clearance [12], but the flight crew should always “trust but verify”, before accepting any clearance.
Especially in extreme latitude operations, the coverage of the TAWS database should be verified, and TAWS modes of operation must be fully understood by the crew.
References:
[1] | EU commission, «Commission regulation 965 (EU-OPS),» 2023. |
[2] | FAA, «14 Code of Federal Regulations 121.354,» 24 10 2024. [Online]. Available: https://www.ecfr.gov/current/title-14/chapter-I/subchapter-G/part-121/subpart-K/section-121.354. [Zugriff am 01 11 2024]. |
[3] | FAA, «AC 25-23 Airworthiness Criteria for the Installation Approval of a Terrain Awareness and Warning System (TAWS) for Part 25 Airplanes,» FAA, 2000. |
[4] | RTCA, «129-20/PMC-2018: Recommended Solutions to Address TAWS Manual Inhibition CFIT Cases Raised by NTSB Safety Recommendations A-17-035 and A-18-015 and GAJSC CFIT WG SE-54,» 2020. |
[5] | Duncan Aviation, «Straight talk about TAWS,» 2014. |
[6] | FAA, «TSO-151c Terrain Awareness and Warning System,» 2012. |
[7] | AIRBUS, «Getting to Grips with Surveillance, issue 2,» 2018. |
[8] | Honeywell, «MK VI and MK VIII Enhanced Ground Proximity Warning System (EGPWS) Pilot's Guide,» 2004. |
[9] | L3 HARRIS, «L3 HARRIS TAWS,» 03 11 2024. [Online]. Available: https://www.l3harris.com/all-capabilities/terrain-awareness-warning-system-taws. [Zugriff am 03 11 2024]. |
[10] | UK AAIB, «Report on the accident to AIRBUS A320-231 G-MEDA on approach to Addis Abeba airport,» 2003. |
[11] | Swedish Accident Investigation Authority, «Final report RM 2013:02e Accident involving a Royal Norwegian Air Force aircraft of type C-130 with call sign HAZE 01,» 2013. |
[12] | ICAO, «Doc. 4444 PANS-ATM, 16th ed.,» 2016. |