Context: Heliports and the 700′ limit

The Hellenic Civil Aviation Authority (HCAA) publishes procedures in its Aeronautical Information Publication (AIP) for helicopters operating from private heliports south of Eleftherios Venizelos (LGAV). A safety study prepared for the Athens air‑traffic service describes the procedures. Helicopter pilots departing or arriving at heliports such as Koropi must cross the ILS localizer beams for runways 03L/03R “as soon as possible at an altitude up to 700 ft” (HCAA AIP – 3.19.21.1.5 and 3.70.21.1.6) to avoid traffic collision avoidance (TCAS) alerts and interference with IFR traffic hcaa.gov.gr. The same study states that inbound helicopters are instructed by ATH TMA Info to contact Venizelos Tower and that the pilots “shall cross without delay the ILS of the runways, not above 700 ft, leaving the area free immediately”and must strictly follow ATC instructions. IFR traffic has priority and continuous two‑way communication is require.

The HCAA’s rule therefore sets a 700‑ft maximum altitude (mean sea level, MSL) for crossing the runway ILS beams. The HCAA (ΑΠΑ) are summoning pilots to testify as they crossed the ILS 3 NM south of the Runway at around 780 ft, roughly 80 ft above this published limit. To judge whether such cases constitute a violation, one must consider the tolerances allowed by instrumentation (altimeter errors) and by operations standards.

Altimeter accuracy according to ICAO PANS‑OPS (Doc 8168)

ICAO Procedures for Air Navigation Services – Aircraft Operating Procedures (PANS‑OPS) are contained in Doc 8168. Volume III, Section 2, Chapter 3 defines the acceptable tolerance for a serviceable altimeter. For altimeters with a test range up to 9 000 m, the permitted error is ±20 m (±60 ft) (Doc 8168, Vol III, Section 2, Chapter 3, Table 2‑3‑1). For altimeters with a test range up to 15 000 m, the permitted error is ±25 m (±80 ft) (Doc 8168, Vol III, Section 2, Chapter 3, Table 2‑3‑2). ICAO further states that if the indication is within tolerance, no adjustment shall be made at any stage of flight and any error within tolerance on the ground should be ignored during flight (Doc 8168, Vol III, Section 2, Chapter 3, Note 1, Note 2 and Note 3 to Tables 2‑3‑1 and 2‑3‑2).

Operational altitude crossings

PANS‑OPS also recognises that altitude crossings are not instantaneous and require defined tolerances in operational procedures. Operators are required to specify crossing altitude deviation tolerances in their SOPs, acknowledging aircraft performance limits and human reaction time (Doc 8168, Vol III, Section 5, Chapter 3, para 3.2(d)).

Altimeter accuracy according to ICAO PANS‑OPS (Doc 8168)

The International Civil Aviation Organization (ICAO) publishes Procedures for Air Navigation Services (PANS‑OPS) in Doc 8168. Volume III covers aircraft operating procedures and includes mandatory pre‑flight altimeter checks. The document requires pilots to set the QNH or QFE and tap the instrument to remove mechanical friction. It then specifies the acceptable error (tolerance) for a serviceable altimeter:

  • When an altimeter is tested on the ground with a range up to 9 000 m (≈30 000 ft), the indicated elevation must be within ±20 m (±60 ft) of the actual height ICAO PANS‑OPS (Doc 8168).

  • For altimeters with a test range up to 15 000 m (≈50 000 ft), the allowed error is ±25 m (±80 ft) ICAO PANS‑OPS (Doc 8168).

  • ICAO notes that these tolerances are considered acceptable for aerodromes up to 3 500 ft elevation, and that if the altimeter reading is within tolerance, no adjustment should be made. Any error within tolerance on the ground should be ignored by the pilot during flight ICAO PANS‑OPS (Doc 8168).

The PANS‑OPS requirement means that even a properly functioning altimeter can legally display a height up to 60 ft (or 80 ft for the wider‑range instruments) above or below the true altitude. ICAO’s guidance explicitly tells pilots not to correct for these inherent errors during flight. ICAO PANS‑OPS (Doc 8168).

Other operational tolerances

  1. Flight‑test tolerances – The Hellenic CAA’s Flight Examiner Manual (which mirrors EASA standards) lists the acceptable deviations during skill tests. For example, during normal flight in a commercial pilot skill test, altitude must be maintained within ±150 ft; for instrument rating tests the tolerance is ±100 ft, and even during simulated emergencies the deviation may be ±200 ft. These tolerances reflect realistic limits for holding altitude, especially in turbulence or when flying manually. A deviation of 80 ft is well within these flight‑test limits.

  2. Instrument system error allowances – In reduced‑vertical‑separation‑minimum (RVSM) airspace, EASA rules accept a maximum assigned altitude deviation (AAD) of ±90 m (±300 ft) and specify that the altitude‑keeping performance should have an altimetry system error of less than ±60 m (±200 ft) (SPA.RVSM.115). Although helicopters operating near LGAV are not in RVSM airspace, these figures illustrate that ATC systems allow significant buffer for instrument error.

  3. Operator procedures – PANS‑OPS Volume III instructs operators to include crossing altitude deviation tolerances in their standard operating procedures for stabilized approaches ICAO PANS‑OPS (Doc 8168). The document does not prescribe a specific figure but recognises that slight deviations are expected and should be accounted for in manuals.

Analysis: Are 80‑ft deviations a violation of the 700‑ft limit?

  1. Nature of the 700‑ft rule – The HCAA AIP requires helicopter pilots to cross the ILS “not above 700 ft” (HCAA AIP – 3.19.21.1.5 and 3.70.21.1.6). This directive is primarily a traffic‑management measure to keep helicopters below the glide paths of IFR arrivals and to prevent TCAS alerts. It is not a published minimum safe altitude; it is a restriction intended to segregate traffic.

  2. Instrument error – ICAO and EASA acknowledge that a serviceable altimeter can indicate a height up to 60–80 ft different from the true altitude ICAO PANS‑OPS (Doc 8168). Therefore, a helicopter pilot whose altimeter displays 700 ft could in reality be flying at 760 ft and still be within the instrument’s certification tolerance. Conversely, a pilot intentionally flying at 680 ft indicated might actually be crossing the ILS at 620 ft. Authorities evaluating altitude deviations must account for this measurement uncertainty.

  3. Pilot workload and human factors – Hovering helicopters in congested airspace must transition from climb or descent to level flight and cross a beam quickly, all while communicating with ATC, monitoring traffic and maintaining situational awareness. Even trained pilots may overshoot the target altitude by tens of feet before stabilising. The Flight Examiner Manual tolerances (±100-150 ft in normal flight) reflect these challenges.

  4. Operational risk – From an air‑traffic‑control perspective, the critical factor is vertical separation between the helicopter and the glide slope of incoming IFR aircraft. An 80‑ft overshoot means the helicopter was still well below 1 000 ft AGL, and IFR arrivals on a 3° glide path would be about 600 ft higher than the helicopter when crossing the same lateral point. The safety study’s objective is to prevent TCAS alerts; minor deviations within altimeter tolerance are unlikely to create a hazard. The study itself recognises that the measure is to avoid “unacceptable ATC workload and delays” rather than a hard obstacle‑clearance requirement.

  5. Compliance and judgment – Pilots should aim to respect the published 700‑ft limit and anticipate instrument errors by flying slightly below the restriction when operationally feasible. However, regulatory enforcement should differentiate between intentional disregard and minor overshoots attributable to instrument error or human reaction time. Both ICAO and EASA explicitly instruct pilots not to correct for small altimeter errors ICAO PANS‑OPS (Doc 8168) – (Annex VII to ED Decision 2022/012/R). Punishing pilots for deviations of 60-80 ft, which fall within the allowable error, would contradict international standards.

Theory of verification and ATC authority

From a regulatory and technical standpoint, an alleged altitude violation in controlled airspace can only be meaningfully assessed if the method of verification is clearly defined and legally sound. ICAO does not recognise visual estimation, subjective observation, or post‑event inference as reliable means of altitude determination. In practice, altitude compliance may be inferred only from surveillance-derived data (Mode C or Mode S pressure altitude), pilot statements, or recorded ATC clearances.

Mode C/S altitude reports are based on pressure altitude referenced to 1013.25 hPa, not QNH. Any subsequent conversion to QNH introduces additional uncertainty due to pressure differences, rounding, encoder resolution (typically 25 ft), and altimetry system error already accepted by ICAO PANS‑OPS. As a result, surveillance data alone cannot determine whether a helicopter exceeded a published QNH-based altitude limit by a margin smaller than the certified tolerance of the altimetry system.

Equally critical is the role of air traffic control authority. Under ICAO Annex 11 and standard ATS practice, a pilot operating in controlled airspace is required to comply with ATC clearances and instructions. If an ATC unit explicitly assigns, approves, or tacitly accepts an altitude either by instruction (e.g. “maintain 700 feet”) or by continued radar control without corrective action, the pilot is operating under ATC authority. In such cases, responsibility for vertical separation and traffic management is shared, and enforcement actions cannot be assessed in isolation from the controller’s instructions, approvals, or lack of intervention.

Furthermore, if a controller observes an aircraft momentarily deviating from a target altitude and does not issue an immediate correction, this is operationally interpreted as acceptance of the deviation within tolerable limits, particularly when no loss of separation, TCAS alert, or procedural breakdown occurs. ICAO operational doctrine does not support retrospective enforcement for transient deviations that were observed in real time and managed – or deemed unnecessary to manage by ATC.

Therefore, any allegation of a violation of the 700‑ft ILS crossing restriction must answer two foundational questions before it can be considered valid: by what technically reliable means was the altitude measured, and what altitude clearance or acceptance was in force from ATC at the time. Without clear, correlated evidence addressing both points, the determination of a violation is neither operationally robust nor aligned with ICAO principles of surveillance accuracy, controller authority, and shared responsibility in controlled airspace.

Conclusion

When the 700‑ft ILS crossing restriction is examined through the lens of ICAO PANS‑OPS and EASA operational philosophy, it becomes clear that enforcement cannot rely on a zero‑tolerance interpretation. ICAO explicitly accepts certified altimeter errors of up to ±60–80 ft, instructs pilots to ignore such errors in flight, and requires operators to define crossing altitude deviation tolerances in their procedures. Against this framework, an alleged exceedance of approximately 80 ft cannot be assessed in isolation from instrument tolerance, surveillance uncertainty, aircraft handling dynamics, and ATC involvement.

Crucially, the burden of proof rests with the authority to demonstrate not only the measured altitude with technical reliability, but also that the pilot was operating outside any ATC clearance or acceptance in force at the time. Where a controller had radar contact, issued or maintained an altitude instruction, and did not intervene, the pilot was operating under ATC authority and within the shared responsibility model established by ICAO.

Absent clear, correlated evidence of a deliberate or significant deviation that created a safety risk, treating minor, momentary altitude differences as violations is inconsistent with ICAO doctrine and with the intent of the published 700‑ft procedure.