Eye to Wheel Height (EWH)
From SKYbrary Wiki
Eye to Wheel Height (EWH), for any specific aircraft type, is defined as the greatest expected vertical distance from the pilot’s eyes to the lowest portion of the aircraft, when at threshold crossing in the normal landing configuration at maximum certificated landing weight. The EWH is normally reported for a 3 degree glideslope, but may differ for other glideslopes.
Note that it is of great importance to realise that this is not the physical dimension of the aircraft when at rest on the ground. Rather, Eye to Wheel Height is the vertical distance from the pilot's eyes to the bottom of the lowest wheels, whilst in flight, under the conditions defined in the preceeding paragraph.
EWH is a critical factor when considering both the safe terrain clearance margins whilst on final approach and the threshold crossing height. EWH can vary from values as little as 4 feet on some small aircraft types to over 40 feet on large, widebody aircraft. Failure to take the EWH value into consideration during the approach and landing phase has the potential to compromise terrain and obstacle clearance and could result in the aircraft touching down prior to the runway threshold.
A visual glideslope indicator, most commonly a PAPI, is installed on most IFR runways. In cases where there is a precision approach, the visual system is normally sited in such a way that the visual glide slope is coincident with the electronic glideslope of the instrument approach. In this circumstance, Threshold Crossing Height (TCH) information is readily available and the crew can assess this information to determine if the guidance provides adequate ground clearance for the EWH of their aircraft type. If there is not a precision approach, the visual system is typically positioned to provide a safe Threshold Crossing Height (TCH) for the aircraft type with the greatest EWH that would normally use the runway. This has the potential to lead to ground clearance issues as not all PAPI installations are designed to ensure the same TCH and, as stated above, there is a wide variance in EWH amongst aircraft types.
On a VFR runway, there may, or may not, be a visual glideslope indicator fitted. If there is no PAPI, or similar system installed, the pilot must be cognisant of both the EWH for the aircraft type flown and the appropriate sight picture at threshold crossing to ensure sufficient margins of safety. If a visual indicator is installed, the pilot must ensure that the system design provides an adequate threshold crossing height for their aircraft type. To achieve this, a Minimum Eye Height over Threshold (MEHT) value should be considered. MEHT is defined as the sum of the Eye to Wheel Height (EWH) and the Wheel to Threshold Height (WTH), for the aircraft in question, as depicted in the diagram below. In the diagram, the Approach Corridor is that area within which the pilot will see an "on slope" indication from the PAPI.
To provide both installation guidance to aerodrome operators and safety information to pilots, some NAA's have established "distance from threshold" criteria for PAPI installations, identified as "D" in the diagram above. They have further categorised those installations as follows:
|Category||EWH||WTH||MEHT||Distance from Threshold|
|AP (Abbreviated PAPI)||up to 3m (10')||3m (10')||6m (20')||126.3m (415')|
|P1||up to 3m (10')||3m (10')||6m (20')||122.7m (400')|
|P2||up to 7.5m (25')||4.5m (15')||12m (40')||245.4m (805')|
|P3||up to 14m (45')||6m (20')||20m (65')||408.9m (1340')|
As can be seen from the table, an AP or P1 designated PAPI installation would only be suitable for an aircraft with an Eye to Wheel Height of 3 metres (10') or less whereas a P2 could safely be used by any aircraft with an EWH of up to 7.5m (25'). In the absence of either an electronic or a visual glideslope, an aircraft with an EWH of 14m (45'), on a nominal 3 degree flightpath, would have to use an aim point over 400m down the runway to ensure that the wheels crossed the threshold at a minimum height of 6m (20').
The concept of EWH is a fundamental factor in ensuring that an aircraft crosses the runway threshold at a safe height. EWH values can vary substantially from aircraft to aircraft and not all visual glideslope indicating system, such as PAPI, are positioned such that they are suitable for use by all aircraft types.
Pilots should be aware of the EWH value for all aircraft that they operate.
- Runway Visual Perspective
- Visual Approach Slope Indicator Systems (VASIS)
- Approach and Landing Risks
Accidents and Incidents
- GL5T, Fox Harbour NS Canada, 2007 On 11 November 2007, a Bombardier BD-700 (Global 5000) operated by Canadian charter company Jetport touched down short of the runway at destination Fox Harbour in normal daylight visibility and then directional control was lost and the aircraft exited the side of the runway ending up having rotated 120° clockwise about its fore-aft axis and came to rest approximately 300metres from the threshold and approximately 50 meters from the runway edge. As a result, the co pilot and one of the passengers suffered serious injuries and the other eight occupants suffered minor injuries. The aircraft sustained major structural damage'