Steepest Demonstrated Approach Gradients

[Xnr]

Does anyone know how manufacturers come up with these numbers and if they only pertain to IFR Coupled approaches?

[Nick Lappos]

Xnr,

They are usually determined during the IFR test phase, and are the result of the steepest ILS gradients found, and not the limits of the aircraft. In the spirit of testing limits, what you can demonstrate becomes the limit, even if nothing wrong is noted. Since ILS approaches are seldom steeper than 3 degrees, the limits become 3 degrees. Sort of a circular argument, chicken and egg wise.

In carefully controlled DGPS tests we were able to take an S-76 regularly to 6 degrees of descent when we started at 80 knots, and up to 9 degrees if we started at 70 knots and decelerated to 30 knots over the threshold.

The steepness of the approach is determined by the ground speed during the approach. Logically, if you have zero knots forward speed, you can approach at 90 degrees, can't you? In fact, most VFR approaches end up at about 30 degree descent angles near touchdown (90 degrees in the last 2 feet!)

Nick

[212man]

Interesting question.

I think Nick's answer conveys a lot that isn't generally realised: often limits are not real limits per se, simply the maximum demonstrated during testing.

As a real example of the point in question, the EC-155 has a maximum approach gradient of 4 degrees. What is the (unusually steep) ILS Glideslope at Marignane (Marseille/Provence)? 4 degrees.

Out of interest Nick, how much easier is it to do the tests prior to certification from a paperwork point of view, as compared to amending certificated limits later? Any difference?

[Xnr]

Thanx Nick

So this has nothing to do with autopilot capabilities.

These test approaches that you flew were hand flown?

Cheers

Xnr

[Nick Lappos]

Xnr,
To answer you quite specifically, the autopilot might have a problem beyond its published limits, or it might not. You don't know until you've tried it, but you would know if it were close to a real limit, based on its behavior. If it starts getting slow and sloppy on the glideslope, and if you get to near zero torque (almost autorotation), then the autopilot is about to give up the ghost. Otherwise, most likely the limit is just a max demonstrated (sometimes the RFM is carefully worded as a "max demonstarted" limit).

We flew the DGPS decelerating approaches as full coupled and as flight directed, and only hand flew the ones that did not decelerate. When full coupled, if we entered at 80 knots or faster, and with 9 degrees approach angle, the approach resulted in near autorotation. Otherwise (slower than 75 knots, or 6 and 7.5 degrees) the approaches worked well.

Usually, it is easiest to set a limit right at the beginning, because the aircraft is already instrumented with the right measurements, and everyone is prepped, with the FAA set in "approve" position. It is a bit harder to get things spun up afterward, but still not a big thing, generally.

[heedm]

If you test to a 4 degree gradient, then fly an ILS that is set at 4 degrees, does this mean you can't correct if you find yourself above the glidepath?

The numbers that I've seen here seem very restrictive. For an enroute descent or descending in a hold, I used to descend faster than 1500fpm at 100KIAS which is steeper than 8.5 degrees in still air. Granted, we kept rate of descent lower closer to the ground, but we weren't restricted to anything so benign as 4 degrees. We were even able to slow down and maintain ROD to steepen the approach.

_____
I paused for a think after writing that. When I flew the steep descents IFR, we were very aware of what was happening with our rotor. If it wasn't loaded enough, you could hear when they started autorotating. At that point we would add power. Is this limitation an airframe limitation but one that while the pilot may be able to cope with it, the autopilot isn't programmed to handle it?

Even if that is the case 4 degrees seems really restrictive.

[Xnr]

Thanx Nick

On another topic we are finding ourselves in a bit of a pickle as Transport Canada has restricted some of the helipads we serve to Cat A performance.

Is there any info that you or Dave Wright may be able to pass along with regard to Cat A ground level helipad procedures for the S76A model. I already have the supplement that you guys did for the CAA.

Sorry Nick I have been trying to track down Dave but he is a hard guy to get hold of.

Cheers

Xnr

[Shawn Coyle]

This may be another hold-over from our fixed wing brothers.
They have limits on the glidepath angle they certify, because for them, glidepath and approach and minimums can be a problem.
Basically, if you want more than 4 degrees of glidepath, as a limitation for your instrument approaches (Category 1 minima), then you have to demonstrate the approach at 2 degrees steeper than the glidepath you want.
Ditto for tailwinds - more than 10 knots (the limit on most airplanes, even if not explicity stated it seems), you need to demonstrate an approach with 50% more tailwind than you get cleared for. Want 15 knots of tailwind - demonstrate it at 22.5 knots.
And for both cases in multi-engine airplanes, demonstrate an engine failure at decision height and either land safely or go around.
Why all this fixed wing talk?
Because we often get saddled by the same criteria.
In this case, the glidepath angle should not be a limiting factor, as all the ILS glidepaths are within this limit. (Perhaps Stephenville Nfld is steeper than this, but hey, who goes to Stephenville?)
The problem is going to come when we eventually throw off our chains and get real helicopter approaches, like Nick talked of earlier, and which I was lucky enough to fly - awesome, and why aren't they being put into use?

[Nick Lappos]

Re-reading this thread, I am reminded of the pair of USAF H-53's that were stuck in fog at Eglin AFB. They stopped at a 500 foot hover, IFR, over the field, using the auto-hover coupler, then simply dialed in progressively lower altitudes on the altitude set knob until they could see the ground! This was reported in the aviation press (Rotor and Wing, I think).

That certainly shows the ability of rotorcraft to perform 90 degree approach angles!

Xnr, call or email me on that Cat A stuff. Dave has had the good sense to retire!

Nick