Analysis

Operator add-ons to minima in CDFA approaches not required anymore?

The new All Weather Operation Regulation (No. 2021/2237) has brough about a frequently overlooked change – the operator add-on to 2D/non-precision approach minima flown using the continuous descent final approach (CFDA) technique, also referred to as the derived decision height, has disappeared!

EASA add-ons – a brief history

Since EASA introduced the necessity for commercial operators to fly all formerly non-precision, now 2D approaches, using the CDFA technique, it has also implicitly required them to add a safety margin to the original PANS-OPS derived minima. The reason behind the logic was simple – 2D approaches are designed with minima established as the height of an obstacle plus an obstacle clearance to give an obstacle clearance height that no aircraft should penetrate. The non-CDFA technique (subject to prior approval for CAT operators) assumes that the crew will start levelling off so as not to penetrate such a height. However, the CDFA technique, the standard for CAT operations, assumes that the crew will make decision whether to continue or to go around while the aircraft is in stable descent towards the runway, i.e. with no level-off intentions. If this decision was made at the obstacle clearance height, the aircraft would descend below this height as a result of its momentum. Therefore, the European approach was to require operators to add a safety margin (usually 50ft but not explicitly prescribed by the regulation) to the obstacle clearance height to account for the height loss between the go-around decision and the actual start of climb. The same requirement applied to NCC operations, worded in a very ambiguous way: For a CDFA approach… “the required descent path should be flown to the DA/H, observing any step-down crossing altitudes if applicable. This DA/H should take into account any add-on to the published minima as identified by the operator’s management system and should be specified in the OM (aerodrome operating minima).“ Theoretically, an operator could have identified the need for a zero add-on and use the obstacle clearance height as its decision height, claiming that the airplane/crew combination could achieve a zero height loss when transitioning to go-around climb, but most operators went for a value of 30 or 50ft to satisfy their authorities and avoid lengthy and costly data collection and proving.

Just a little remark – both EASA and ICAO require operators to establish a so-called minimum descent height (for 2D operations) that is not lower than the obstacle clearance height, but may include a safety margin to account for specific airplane/crew capabilities (not to) capture and maintain the obstacle clearance height when approaching the instrument approach minima. However, historically, most operators claimed this add-on to be zero, i.e. stated that their aircraft/crews could approach the obstacle clearance height and maintain it with no altitude bust. So the CDFA add-on or derived decision height were intended to be added to the minimum descent height, but practically were added to the obstacle clearance height as it equalled the operator established minimum descent height.

For SPO, the text differed and did not require any add-on, while rather inconsistently stating that the decision height of a CDFA 2D approach should not be lower than any published DH – which would never be published for a then non-precision approach as per the valid instrument approach design rules.

No add-ons now?

With the new AWO regulation, the requirement for an add-on or derived decision height has disappeared. The above cited text has been removed and a new guidance material added (ref. GM5 CAT.OP.MPA.110, GM7 NCC.OP.110, GM8 SPO.OP.110). This GM advocates the use of the minimum descent height as the decision height for a CDFA approach – saying that it is safer than using a non-CDFA step down approach, but interestingly enough not saying anything about the comparison to the previously used and now common derived decision heights. This appears to be a blanket abolition of the CDFA add-ons. However, watch out! The same GM continues with the following: …“it is necessary for operators to assess whether their cockpit procedures and training are adequate to ensure minimal height loss in case of a go-around manoeuvre.“ Therefore, to apply for a zero add-on (CAT operator’s minima are subject to authority approval, NCC and SPO operators only have to publish the method in their operations manual), the operator must be prepared to present a safety assessment that proves the above mentioned minimal loss. What is considered to be minimal by the authorities remains unclear.

What about general aviation?

The old provisions for NCO did not mention CDFA in any way, therefore it was at the discretion of the pilot to use a convenient and appropriate technique for non-precision approaches. The new AWO regulation gives NCO pilots the liberty to choose whether a 2D approach will be flown using the CDFA or step-down fix technique, citing the benefits of CDFA in commercial and complex-aircraft operations but also adding that general aviation pilots might be more familiar with steeper approaches and hence might prefer step-down descents. GM1 NCO.OP.111 explains that there is no minimum descent height for CDFA approaches (…while in CAT, SPO and NCC there is one and you can use it instead of the former derived decision height…) and a brief descent below the decision height is acceptable. A different paragraph requires such a decision height not be lower than the obstacle clearance height, so basically EASA admits that a brief descent below the obstacle clearance height is acceptable and safe.

To add or not to add…

You are probably asking what the outcome of all this is. For CAT, SPO and NCC operations, CDFA add-ons are not mandatory, however, if you do not wish to use them, you must be able to prove a safety assessment demonstrating minimal height loss during a go-around. Operators can revert back to the minimum descent height using it as a decision height, i.e. in continuous and stable descent down to the runway instead of levelling off. If you cannot demonstrate the safety assessment, keep using the add-on. For NCO, no safety assessment is required, and pilots may use the published obstacle clearance heights (provided they are consistent with aircraft and system minima) directly as their decision heights.

Also note that there are inconsistencies in the terminology, while in NCO, it is clearly stated that there is no minimum descent height in CDFA approaches and this concept has long been nurtured in CAT/SPO/NCC operations, the EU together with EASA seem to be unable to get rid of this term to remain consistent with ICAO, so references to the minimum descent height in relation to CDFA operations are still commonplace. However, it is generally understood that irrespective of the term used, the operator has to publish minima at which the pilot does not arrest the descent and only makes the decision to continue or go around while flying a stable approach.

Safety aspects

The most important consideration, of course, is safety. The minimum obstacle clearance applied to 2D/non-precision approaches is at least 75m in the final segment. What is the risk compared to a 3D approach? The design vertical separation between the obstacle and the nominal path there equals the height loss, which ranges between 40 and 49m (for airplanes) depending on their speed. In a 3D approach, the assumption is that the aircraft will use all this height to transition to climb and will come really close to the obstacle. Looking at this, there are 75m in a 2D approach, but the aircraft should not lose more than 40 to 49m anyway, as the initiation of the climb is no different from a 2D or 3D approach. Therefore the aircraft will not get closer to an obstacle than it would do in a 3D approach, even if the obstacle clearance height is used as the decision height.