Standard vs Non-standard Phraseology – differences between ICAO Level 4 and Level 6

In the previous post you learnt about the importance of ICAO Standard English Phraseology and its influence on Aviation Safety – click here to check it!

The safety takeaway from the article is that Most aviation accidents are not caused by lack of English — but by lack of standard English and that:

If a message is important, it must be standard.

If it is not standard, it must be clear.

By all means, it is not about sounding professional — it is about being understood correctly, every time .

In this article, we would like to show you, though, that “Standard” is Not “Optional” and even if we use standard, there are differences between how we express ourselves, how we convey the messages and how clear they are for our listeners. The more advanced your language, the higher your ICAO LEVEL.

In an emergency, the human brain tends to revert to its native language or “plain English.” Training in ICAO phraseology creates muscle memory. By making the language “robotic,” aviation professionals ensure that even under extreme stress or oxygen deprivation (hypoxia), the communication remains functional and unambiguous .

Let’s look at the practical comparison examples between ICAO LEVEL 4 vs LEVEL 6.

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1. ROUTINE SITUATION — TAXI CLEARANCE CONFUSION

Situation

The pilot is unsure about the assigned taxi route.

ICAO LEVEL 4

“Uh… we are not sure about the taxiway. Can you say again?”

Characteristics:

• Meaning is understandable
• Hesitation and filler words
• Limited precision
• Reactive, not proactive
• ☑ Operationally acceptable
• ❌ Not polished or safety-optimal

ICAO LEVEL 6

“Confirm taxi route to holding point runway two seven via Alpha.”

Characteristics:

• Clear and concise
• Correct structure
• No ambiguity
• Uses standard phraseology
• ☑ High situational awareness
• ☑ Professional and efficient

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2. ABNORMAL SITUATION — WEATHER AVOIDANCE

Situation

Pilot needs deviation due to weather.

ICAO LEVEL 4

“We have weather ahead and need to go a little right.”

Characteristics:

• Message is clear
• Non-standard phrasing
• Vague distance and intent
• ☑ Message understood
• ❌ Limited detail

ICAO LEVEL 6

“Request ten degrees right for weather avoidance.”

Characteristics:

• Specific
• Standard ICAO structure
• Predictable phrasing
• ☑ Easier for ATC to respond
• ☑ Reduced misunderstanding

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3. NON-STANDARD SITUATION — UNABLE CLEARANCE

Situation

Pilot cannot comply with altitude clearance.

ICAO LEVEL 4

“We cannot climb now because the aircraft is too heavy.”

Characteristics:

• Meaning clear
• Extra words
• Informal explanation
• ☑ Acceptable
• ❌ Inefficient

ICAO LEVEL 6

“Unable to climb due to aircraft performance.”

Characteristics:

• Short and standard
• Correct use of “unable”
• Professional register
• ☑ Ideal ICAO response

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4. EMERGENCY-RELATED — TECHNICAL PROBLEM (NO MAYDAY)

Situation

Minor technical issue, no immediate danger.

ICAO LEVEL 4

“We have a small problem with the engine but it is okay.”

Characteristics:

• Vague
• Reassuring but unclear
• Poor information quality
• ❌ Risk of misunderstanding

ICAO LEVEL 6

“We have an engine indication abnormality, request to maintain present level.”

Characteristics:

• Precise
• Non-emotional
• Action-oriented
• ☑ Supports ATC decision-making

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5. ACCIDENT / INCIDENT DISCUSSION (EXAM PICTURE TASK)

Situation

Candidate describes an aircraft damaged after runway excursion.

ICAO LEVEL 4

“The plane is damaged and it went out of the runway. Maybe the weather was bad.”

Characteristics:

• Basic description
• Limited vocabulary
• Simple speculation
• ☑ Minimum acceptable

ICAO LEVEL 6

“The aircraft appears to have overshot the runway after landing. The damage suggests a loss of directional control, possibly influenced by contaminated runway conditions.”

Characteristics:

• Accurate observation
• Controlled speculation
• Safety-focused language
• ☑ Examiner-level response

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6. CRM / DECISION-MAKING QUESTION

Situation

Crew discussion after unstable approach.

ICAO LEVEL 4

“Why did you continue? It was not stable.”

Characteristics:

• Direct but confrontational
• Limited CRM tone
• ⚠ Understandable, but risky

ICAO LEVEL 6

“What factors influenced the decision to continue the approach?”

Characteristics:

• Neutral
• CRM-appropriate
• Encourages discussion
• ☑ Ideal for safety culture

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7. KEY DIFFERENCES — QUICK EXAM SUMMARY

Feature	ICAO Level 4	ICAO Level 6
Clarity	Adequate	Precise
Vocabulary	Limited	Aviation-specific
Structure	Basic	Predictable
Fluency	Some hesitation only	Natural & controlled
CRM tone	Neutral / direct	Professional & non-blaming
Safety impact	Acceptable	Optimal

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PART 2 — ADAPTATION FOR ATC

Now let’s reuse the same structure, but shift the perspective.

(Controller ↔ Pilot)

Key difference

• Pilots: compliance + clarification
• ATC: clarity, sequencing, anticipation

Pilot: “Request taxi.”

ICAO LEVEL 4 ATC:

“Taxi to runway two seven.”

⚠ Issues:

• missing holding point
• no taxi route

ICAO LEVEL 6 ATC:

“Taxi to holding point runway two seven via Alpha.”

☑ Complete ☑ Predictable ☑ Safe

ATC-SPECIFIC MARKING ADDITIONS

1. INFORMATION STRUCTURE (CRITICAL FOR ATC)

Level	Examiner indicators
4	Correct but incomplete clearances
6	Complete, sequenced, unambiguous

Listen for:

• runway number included
• route included
• restrictions included

2. ANTICIPATION & MONITORING

Level	Examiner indicators
4	Reacts to pilot requests
6	Anticipates pilot needs

Example:

• Level 6 ATC adds: “Expect further clearance at holding point.”

3. ERROR TRAPPING

Level	Examiner indicators
4	Corrects errors when obvious
6	Detects incorrect readbacks immediately

ATC INCIDENT DISCUSSION (for example PICTURE TASK)

ICAO Level 4

“The aircraft is damaged. Maybe it’s the pilot’s mistake.”

❌ Blame-focused

❌ Weak safety value

ICAO Level 6

“The image shows significant nose damage. Runway conditions and operational factors may have contributed.”

☑ System thinking

☑ Professional neutrality

ATC EXAMINER GOLDEN RULE A Level 6 controller does not just give clearances — they manage traffic through language.

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Commonly Misunderstood Phrases in Aviation

Here is a breakdown of commonly misunderstood phrases, the risks they carry, and the ICAO Standard equivalents that keep the skies safe.

1. “Roger”

❌ Common misuse: “Roger” (used to mean yes / cleared / will do)

Risk:

• Does NOT mean compliance
• Has caused aircraft to enter runways or descend without clearance

ICAO Standard:

• Use “Wilco” → I understand and will comply
• Or full readback of the clearance

Example (safe): “Cleared to land runway two seven, ABC123.”

2. “Okay” / “Alright”

❌ Common misuse: “Okay, descending.”

Risk:

• Informal
• No legal meaning
• Can hide misunderstanding

ICAO Standard:

• Use “Descending”, “Climbing”, “Maintaining”

Example: “Descending to flight level one zero zero.”

3. “Takeoff” (used incorrectly)

❌ Common misuse: “Ready for takeoff.”

Risk:

• Correct only when actually clearing for takeoff.
• Runway incursions
• Fatal accidents (e.g. Tenerife)

ICAO Standard:

• Before clearance: “Ready for departure”
• Clearance: “Cleared for takeoff” (only ATC)

4. “At your discretion”

❌ Common misuse: “Descend at your discretion.” (without limits)

Risk:

• Different interpretations
• Uncontrolled descent profiles

ICAO Standard:

• Always include a limit

Example: “Descend at pilot’s discretion to flight level one eight zero.”

5. “As soon as possible”

❌ Common misuse: “Climb as soon as possible.”

Risk:

• Subjective timing
• Conflicting expectations between pilot and ATC

ICAO Standard:

• Use specific instructions

Example: “Climb now to flight level two four zero.”

6. “Continue” (Approach & Landing)

❌ Common misuse: “Continue.”

Risk:

• Pilot may interpret as landing clearance
• Contributed to approach and runway accidents

ICAO Standard:

• “Continue approach”
• “Cleared to land” only when appropriate

7. “Stand by”

❌ Common misuse: “Stand by.” (no follow-up)

Risk:

• Pilot waits indefinitely
• Critical time lost in abnormal situations

ICAO Standard:

• Use sparingly and re-contact quickly
• Or give conditional instruction

8. “We are good” / “No problem”

❌ Common misuse: “Everything is good now.”

Risk:

• Hides technical or operational issues
• ATC loses situational awareness

ICAO Standard:

• State status clearly

Example: “Technical issue resolved, able to continue.”

9. “Can you…?” instead of “Request…”

❌ Common misuse: “Can you give us lower?”

Risk:

• Sounds informal
• Weakens urgency
• Unclear operational intent

ICAO Standard:

• “Request lower altitude.”

10. “I think / maybe / probably”

❌ Common misuse: “I think we are on the wrong taxiway.”

Risk:

• Hesitation
• Delayed corrective action

ICAO Standard:

• “Confirm taxiway.”
• “We may be on incorrect taxiway.”

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Summary Table (Training-Friendly)

Non-Standard Phrase	Risk	ICAO-Standard Equivalent
Roger	No compliance	Wilco / readback
Okay	Informal	Descending / Climbing
Ready for takeoff	Runway incursion	Ready for departure
As soon as possible	Ambiguity	Climb now
Continue	Landing confusion	Continue approach
Can you…	Weak request	Request…
We’re good	Hidden risk	Status stated clearly

For your information, please find more comparisons between level 4 and 6.

Task type: Routine / non-routine operational exchange

1. PRONUNCIATION

Level	Examiner indicators
4	Accent noticeable but intelligible; occasional repetition required, misunderstandings occurred
6	Clear, natural pronunciation; no effort required to understand, no misunderstandings

2. STRUCTURE & PHRASEOLOGY

The examiner listens for:

• correct verb use (request, confirm, unable)
• correct number format
• proper readbacks

Level	Examiner indicators
4	Meaning clear, but wording sometimes non-standard, non-standard but acceptable
6	Consistent use of ICAO phraseology and correct structure, standard and precise

3. FLUENCY

Level	Examiner indicators
4	Some hesitation, fillers, reformulation, hesitation affects flow
6	Smooth delivery, appropriate pace, natural operational rhythm

4. COMPREHENSION & INTERACTION

Key check: Does the candidate target missing information?

Level	Examiner indicators
4	Responds correctly after clarification, vague clarification
6	Anticipates, clarifies proactively, precise clarification

5. SAFETY IMPACT (Examiner Judgment)

Final examiner note: Does the candidate’s language support safe operations under pressure?

Level	Examiner indicators
4	Communication adequate but not optimized
6	Communication actively reduces risk

PICTURE DESCRIPTION / INCIDENT DISCUSSION

1. DESCRIPTION ACCURACY

Level	Examiner indicators
4	Describes main elements, limited detail, observations mixed with assumptions
6	Accurate, structured, relevant details, observations clearly separated

2. SPECULATION CONTROL

Level	Examiner indicators
4	Uses “maybe”, “I think” frequently, uncontrolled speculation
6	Uses controlled phrases (appears to, suggests), professional speculation

3. VOCABULARY RANGE

Level	Examiner indicators
4	General aviation terms
6	Precise technical vocabulary

4. CRM / SAFETY TONE

Level	Examiner indicators
4	Neutral, sometimes direct, personal judgment language
6	Non-blaming, system-focused, safety-system language

OVERALL ICAO DECISION SUPPORT

Examiner guidance:

• Level 4 = meaning achieved
• Level 6 = risk managed through language

Follow us for more differences between levels and requirements you must meet to reach level 5 or 6.