The ‘PULL UP’ audio and visual ‘Hard’ warning generated by TAWS or GPWS systems is an absolutely vital safety net. Every pilot should understand that their response should be rapid and instinctive. Such a callout is not a time to ask the question 'why?' or to ever assume that the activation might be a false one.
Provided that the database it contains is kept up to date, and the equipment is provided with accurate position data by internal or external GPS feed, any correctly functioning TAWS system will provide a reliable protection against inadvertent closure with terrain, or an obstruction, provided that the response is appropriate.
Since the older GPWS systems do not have a position feed or a terrain database, the ‘PULL UP’ Warnings which they generate may not be enough to prevent terrain impact even if actioned properly. However, an appropriate response still represents the best course of action and the most significant chance of survival.
The Standard Flight Crew Response to a ‘PULL UP’ Warning
The appropriate response to a TAWS or GPWS ‘PULL UP’ warning for a particular aircraft type will be found in the AFM or Pilot Operating Handbook. This will be a Memory Drill and will be replicated in the Operations Manual in the case of an aircraft employed in public transport.
It is worth noting that the standard calls/callouts and the sequence of actions may differ in the following, seemingly similar situations:
However, the principles of any such response will be found in all such drills. The promulgated escape manoeuvre will aim to ensure the establishment of the maximum angle of climb. It must be initiated immediately with the following sequence of actions:
- Disconnect the Autopilot and Auto-throttle if engaged (may not be required for some aircraft for the Go-around case);
- Advance the Thrust or Power levers to the Take Off position;
- Rotate the aircraft to a pitch up angle which reaches the margin of stick shaker activation. This is likely to be around 20 degrees pitch up for most aircraft;
- If the Landing Gear is selected down, do not raise it immediately (see Notes below);
- If speed brakes are deployed, retract them;
- If trailing edge Flaps are deployed, do not retract them immediately (see Notes below).
Notes: Selecting the landing gear up initially results in a loss of climb gradient capability due to the drag created by the opening of the landing gear doors; the retraction of the landing gear should therefore be delayed until after a safe flight path has been achieved, the GPWS/TAWS warning has ceased, the speed is above the minimum selectable speed or stick-shaker speed and a positive vertical speed has been achieved.
The retraction of one step of flap should also be delayed as above.
The maximum climb angle achieved must be maintained until the flight crew are sure that the aircraft is above the applicable Minimum Sector Altitude or until it is visually apparent, without any doubt, that the aircraft is clear of terrain and obstacles. The risk that pressure altimeters might have been mis-set or that the aircraft may not be in the position previously expected should be born in mind when determining that these conditions have been met.
Some Fly by Wire aircraft types may have systems for automatic acquisition of a maximum angle of climb, such that the generic sequence above may not apply. Reference should always be made to the memory drill contained in the AFM for the aircraft type.
More generally, some Operations Manuals may permit an exception to the standard response if it is possible to instantly recognise that certain criteria are met at the point at which a ‘PULL UP’ Warning occurs. These are usually if:
- The aircraft is definitely above MSA at the time the warning occurs
- The aircraft is on final approach below 1000 ft aal, with the landing runway in sight, the landing checklist has already been completed, the aircraft is stabilised on the correct vertical profile, and in all other respects it is immediately obvious to both flight crew that their aircraft is not in danger because of it’s configuration, proximity to terrain or flight path.
Unservisability of TAWS / EGPWS Equipment
It is common for an aircraft MEL to allow despatch with an unserviceable TAWS or GPWS system. If this is the case, the MEL entry often specifies that ‘alternative procedures are to be established and used'. These may be fully specified in an associated Operations Manual or that Manual may simply give guidance on the action to be taken according to the specifically intended flight. This may include prohibition of flights into and out of particular airports under certain meteorological conditions or by night.
Reporting of TAWS/GPWS ‘PULL UP’ Warnings
All flight crew who receive a ‘PULL UP’ warning should raise an Air Safety Report (ASR) detailing all known circumstances. ATCOs who become aware of such occurrences should do likewise and their organisation should then advise the Operator. Aircraft Operators with a flight data monitoring programme should have an automatic alerting system for any recording which contains such an occurrence, and this should support an investigation already initiated by receipt of an ASR or prompt the commencement of a new investigation.
Related Accident & Serious Incident Reports
On 10 April 2010, a Polish Air Force Tupolev Tu-154M on a pre-arranged VIP flight into Smolensk Severny failed to adhere to landing minima during a non precision approach with thick fog reported and after ignoring a TAWS ‘PULL UP’ Warning in IMC continued descent off track and into the ground. All of the Contributory Factors to the pilot error cause found by the Investigation related to the operation of the aircraft in a range of respects including a failure by the crew to obtain adequate weather information for the intended destination prior to and during the flight.
On 15 April 2002, a Boeing 767-200 attempting a circling approach at Busan in poor visibility crashed into terrain after failing to follow the prescribed procedure or go around when sight of the runway was lost. 129 of the 166 occupants were killed. The Investigation attributed the accident to actions and inactions of the pilots but noted that the aircraft operator bore considerable contextual responsibility for the poor crew performance. It was also concluded that ATC could have done more to manage the risk procedurally and tactically on the day and that ATM regulatory requirements did not adequately address risk.
On 31 March 2003, an A320, operated by British Mediterranean AW, narrowly missed colliding with terrain during a non-precision approach to Addis Ababa, Ethiopia.
On 18 January 2006, a Dornier 328 on descent into Manchester UK, avoided CFIT only by response to EGPWS following failure to capture the ILS Glideslope and a high rate of descent in IMC.