RT Discipline
 

-Collected and Contributed by A.Sanyal, DGM (ATC), Ahmedabad

Safety is no Accident or Incident. Air Traffic Control with adherence to proper RT discipline and proper procedures must be used to increase safety in the skies. Use of standard phraseology prevents misunderstanding and language difficulties particularly where English is not the pilot’s mother tongue. To underline the need of proper RT discipline a few case studies are elaborated for the benefit of working controllers.

 

 

                                             

 

 

Incident -1

An ATC error placed two aircraft in confliction with each other and the controller was late in recognizing the developing situation. When the controller became aware of the confliction he gave prompt heading and level instructions to resolve it but in a manner of delivery which would be used for routine communications. This resulted in the aircraft’s manoeuvre rate being insufficient to provide adequate separation and an AIRPROX occurred. Use of the phrase ‘Avoiding Action’ plus the provision of ‘Traffic Information’ would have allowed the pilots to react expeditiously and may have assisted in early visual contact being established. Visual acquisition can help provide the crew with the means to ensure that separation is sufficient to prevent a mid-air collision.

RT Causal Factors

When taking action to resolve the situation, ATC did not use standard phraseology that would immediately alert pilots to take immediate avoiding action.

Incident-2

A foreign ATC unit cleared an aircraft for descent and a procedural approach using the phrase ‘Descend two four zero zero, cleared for NDB approach’. It was night, there was no radar available, and the flight was following a procedural approach which commenced at the NDB at 2400 feet AMSL. The pilot read back ‘OK, four zero zero’. Playback of the cockpit voice recorder indicated that the pilots received a momentary GPWS warning and the aircraft was destroyed, killing all on board. The impact points was 1 NM before the NDB and 8 NM from touchdown.  

RT Causal Factors

  • The pilot misheard this as a clearance to descend ‘to’ 400 feet AMSL.

  • The pilot’s readback was non-standard.

  • ATC did not hear the incorrect readback and so failed to correct the error.

Incident-3

A foreign ATC unit cleared an aircraft for descent using the phrase ‘Re-clear to three thousand feet …. (pause)…. Expect an ILS approach report level at three thousand feet’. There was no radar available and the pilot was flying in intermittent IMC. The pilot read back ‘re-cleared to two thousand feet’ however this transmission commenced during the pause in the ATC transmission and was missed by the controller. ATC did not query the lack of a readback from the pilot. In addition, although not a RT factor, ATC had also passed an incorrect QNH value, which placed the aircraft 240 feet lower than was indicated on the aircraft altimeter. The playback of the cockpit voice recorder indicated that the crew took action to level at an indicated 2000 feet AMSL and almost instantaneously the aircraft received a GPWS warning. This was 7 seconds before impact as the aircraft passed through an actual altitude of 1800 feet AMSL. No action was taken on the GPWS warning and the aircraft crashed into a mountainside at 1795 feet AMSL, only 100 feet below the summit. All 144 persons on board were killed.

RT Causal Factors

  • The pilot misheard this as a clearance to descend to 2000 feet AMSL.

  • A pause during a continuous ATC transmission meant that the pilot’s incorrect readback, which took place simultaneously, was not heard by ATC.

  • ATC did not query the lack of a readback from the pilot.

Incident-4

Aircraft ‘A’ was climbing on a Standard Instrument Departure (SID) within busy TMA airspace, initially to 6000 feet AMSL. Aircraft ‘B’ was descending to FL90 inbound to a TMA airfield and conflicted in plan with the departing aircraft. To establish separation which would allow continuous climb and descent for the subject aircraft, ATC cleared aircraft ‘A’ to ‘Head one hundred degrees and climb Flight Level eight zero’. The pilot read back ‘One zero zero and Flight Level eight zero’. Subsequently aircraft ‘A’ was noted on radar by ATC climbing through FL80 and confirmation of his level was sought by the controller. The pilot reported ‘We were cleared climb one zero zero’. Aircraft ‘A’ was instructed to stop its climb immediately at FL90 and aircraft ‘B’ was instructed to stop descent at FL100. However due to the late call and the fact that radar updates lag behind an aircraft’s true vertical position, aircraft ‘B’ was unable to arrest its descent until FL93. Specific avoiding action was then given and the aircraft passed with less than 1 NM horizontal and only 300 feet vertical separation. The subsequent AIRPROX investigation also revealed that the ATC controller’s RT discipline leading up to the incident was generally poor with regular omissions of key phrases such as ‘to Altitude xxxx’ when passing level instructions, e.g. he said ‘Descend one thousand feet’ and ‘Climb six thousand feet’. Phrases such as these could be open to misinterpretation, particularly where English may not be the pilot’s first language. The pilot could interpret that ATC require him to change his height or altitude by an amount rather than fly to a cleared height or altitude.

RT Causal Factors

  • ATC used the phrase ‘One hundred’ when passing a heading instruction. (‘One hundred’ must only be used for Flight Level instructions.)

  • Although the pilot of aircraft ‘A’ read back the clearance in the correct format, the second pilot on board erroneously set the autopilot’s Flight Level / Altitude selector to FL100, probably due to association of Flight Level one hundred with the incorrect phraseology used by ATC to give vectoring instructions.

  • When taking action to resolve the situation, ATC did not use standard phraseology that would immediately alert pilots to a deteriorating situations.

                                

Ways to improve RT discipline

  • Let us aim for accurate, brief and clear transmissions.

  • Let us LISTEN carefully and just not HEAR what we expect to HEAR.

  • Before transmitting anything, let us learn to listen out first. Let us not interrupt a dialogue or block another transmission.

  • On first contact with an ATC center, pilots must report their actual flight level / altitude and cleared flight level / altitude if different. Such a report provide ATC with a safety check, a level verification and allow other airspace users to build up situational awareness.

  • Let us pass all instructions and clearances in clear and unambiguous manner using standard phraseology.

  • Let us try to limit the number of instructions passed in any one transmission to a maximum of three-ideally only two if practicable. Where there are large amount of numbers to be passed let us speak clearly and slowly.

  • The mandatory items that pilots need to always read back are –

 

QTaxi instructions

QLevel instructions

QHeading instructions

QSpeed instructions

QAirways or route clearances

QRunway-in-use

QClearances to enter, land on, take-off on, backtrack, cross, or hold short of an active runway.

QSSR operating instructions

QAltimeter settings

QVDF information

QFrequency changes

QTypes of radar service

 

If we are in doubt about any transmission receive, or do not receive an expected read back, then we should immediately demand a read back from pilot for checking.