When we are flying around, unless you are flying old school IFR at very low level (I Follow Roads), there are no posted speed limit signs. There are some speed limits with respect to weather limits in the Rules of the Air but when you fly in cloud they do not apply and we can just go flat out. At least as long as we respect the 250 kt limitation below 10000 ft. Or can we?

To comprehend the situation in IFR as pilots, we need to understand what the instrument procedure designer was making assumptions about when they designed the procedures we use. The underlying procedures for the creation of instrument procedures such as SID, STAR and approaches are contained in ICAO Doc 8168 PANS OPS which comes in the 3 volumes. Helpfully, thanks to the Swiss, available here. There is military add on called AATCP-1 but the core is the same.

The first volume is all about how the procedures should be used, the second about how they should be designed and the third is information for pilots and controllers. However, whilst that would suggest pilots should pickup only Volume 3, that leads to missing large chunks of knowledge. Volume 1 and 2 are critical and should form part of every IFR pilot’s education and ongoing professional development.

One crucial aspect of IFR approach operations is the Category; aircraft speed is the most influential part of an aircraft’s performance with respect to procedure design and will alter the envelope of airspace that must be considered when assessing obstacles. Practically for pilots this changes your minima and often your outbound track for a procedure turn.

For fixed wing aircraft, the category is selected based on V at which is “Speed at threshold” but more accurately 1.3 x stall speed or 1.23 x stall speed in approach configuration at maximum weight. The breakdown is below:

Since helicopters do not stall in the same way, they can use Category A (the slowest category) or use their own special “H” category. Why is a helicopter special though? That is the critical question.

A quick analysis of the differences between charts for the different categories reveals what IFR pilot’s are familiar with – different approach minima for each category, different outbound headings for procedural turns and perhaps different timings for those turns and holding procedures.

The chart above shows Cat A, B and C with significant differences in OCH, outbound headings and circling minima (remember of course OCH is only the starting point of the DA/DH calculation).

Again, why are they different? The obvious answer is by looking at how we chose a Category in the first place – stall speed. An aircraft in a higher category flies faster during all phases of an approach. This critically means they have more downwards inertia to reverse at the DA/DH and they will dip lower in the go around. There is also the assumed turn radius for the aircraft and hence the different turn radius during a teardrop and procedural turn.

Focussing now on helicopters, there is no stall speed so they are assumed to be category A or in the special category H. They also have the luxury of being able to fly at any speed between about 30 kts backwards, all the way up to VNE. They usually have a greater ability to initiate a climb at DA/DH without losing significant heights (save when OEI in most helicopters). Thus they have more flexibility and can choose for example to expedite an approach at 120 kts one day or slow it right down to 30 kts on another day when visibility is poor (perhaps only with a decent AFCS).

Except that is not what the procedure designer expects when they are assessing clearance from obstacles. The procedure designer has a set of speeds they assume you will fly each phase of the approach including the hold, initial approach and missed approach. If you deviate from those figures, you could completely invalidate the assumptions made about your path over the ground, the turn radius you will achieve during the missed approach, the amount of undershoot of DA/DH you will make in the worst case and the space your holding will take up in the air.

This has been considered to some degree in the PANS OPS document in that it says for fixed wing aircraft, the same aircraft may have different categories at different masses or the crew can choose to operate in a different category. However, this is not really covered for helicopters.

The elephant in the room with this discussion is that the designers assumptions on speeds are not shown at all on the chart. Only when the designer deviates from the assumptions will a speed be mentioned. Take the approaches at Bristol Airport, UK. There is a tiny note (Note 2) at the bottom which states the speed limit for the procedure is 110 kts for Cat A. You will not find such a note on most procedures but there is an assumed limit by the designer! It would nice to know what it is!!

Here is the rest of the table from PANS OPS:

For helicopters, first look at Category A. The range of speeds for reversal procedures is the first critical one – it’s 110 kts. Thus when you are going outbound for a procedure for a teardrop turn, the designer has assumed you are only going at 110 kts. The obstacle clearance in the turn back onto finals assumes this speed and thus if you are actually pushing 150 kts your turn radius will be much wider possibly leading to reduced obstacle clearance and an overshoot of the final track. The designer of course also has a set of assumed winds so the margins are probably big enough. The timing type reversals will also be thrown off.

The approach speed range is next: 70 – 100 kts. If you blast down the approach to expedite at 150 kts you are significantly changing the game – how much margin is left? Finally looking at the missed approach, the restrictions are 100 kts until the first turn then 110 kts. Many helicopters AEO can outperform this for a reasonable rate of climb. But how wide is your turn in the missed approach phase? Conversely if you are really slow (say 50 kts) could your turn be so tight that you turn into something near the airfield (you should of course not initiate any turn until at least the Departure End of the Runway (DER) for exactly this reason).

The difference might be even more marked for Cat H. An approach or missed approach at 150 kts is 66% faster than the assumption. That is a big margin!!

However, there is a “So what?”. Many charts are drawn for Cat A-D (or even E) so the designer has considered speeds way above the limit of a conventional helicopter. Phew. No problem.

Except some publishers (eg Airbox in the UK before they withdrew from the instrument procedure game) and individual charts only show Cat A and/or Cat H, even if the underlying AIP version has other categories. You have no information about what your excessively high or low speed does to the designer’s assumptions for Cat A. What obstacles or eventualities have they disregarded at Cat A because they assume you are at a certain speed? You could always check the AIP for the other Categories but that’s not practical in the air.

But what if there never have been other Cat’s as in the Cat A only Lands End example below; in this case there is a sharp turn after the Missed Approach. The designer assumes you will fly this turn at no more than 100 kts before accelerating up to 110 kts. If you are flying at 150 kts will you get close to the terrain in the wider left turn? If you fly right back at 50 kts, how close to the mast will you get? Probably not that close, but assumptions are a hole in the swiss cheese, particularly when IMC, you have just executed a go around and you’re thinking “Where do I divert?”.

And you at the back, yes you the military helicopter pilot. What about COPTER procedures in your military terminal approach procedure books? Unfortunately, there is the same issue. Whilst ICAO PANS OPS describes most approaches, the military have an addendum in the form of AATPC-1 which describes “COPTER” procedures, TACAN approaches and PAR alongside procedures for fast jets (aka High Performance Military Aircraft – HPMA). In AATPC-1 it states:

In Chapter 2:

2.1-1 Criteria in addition to the helicopter (CAT H) criteria in PANS-OPS and this supplement are specified in chapter 8 of this publication.

2.1.10-3 Procedure identification for helicopters

Helicopter only procedures shall bear an identification that includes the term “COPTER”. For example: COPTER VOR 090, COPTER TACAN RWY 27.

Then in Chapter 8:

8.2.1 Application.

These criteria are based on the unique manoeuvring capability of the helicopter at airspeeds not exceeding 90 kt on final approach and missed approach.

This clearly highlights that the designer only considered a maximum airspeed on approach of 90 kts. Anything faster is a voyage of discovery.

Summary

Instrument approach procedure designer make a whole raft of assumptions about how pilots will fly them. It is vital to comprehend this underlying point and apply it when operating. Often making an approach at a speed outside the designers assumptions will probably not cause any issue. I think probably is not good enough in our safety-focussed aviation world.

A little summary of some critical speeds for your next approach from the table:

• Cat A approach – 70-100 kts
• Cat H approach – 60-90 kts
• Cat A missed approach – 100 kts until first turn then 110 kts
• Cat H missed approach – 90 kts

Take a moment to read PANS OPS (at least Volume III).