SCDA Approaches

[North Shore]

Can anyone explain an SCDA approach to me, please?

The best I can think of is that it's a procedure to make a non-precision approach more like an ILS - that is, rather than stepping down, you keep up a constant descent rate/angle.. but then what happens at the bottom? Do you get down to MDA right at the MAP, and so if you don't see anything, you go around, or do you get down to MDA prior to the MAP?

What do the plates look like? (Can anyone post one, please?) or is it an in-flight calculation that gets you started from top of descent?

Are they company generated approaches? Or does transport have their hands in there, too?


(This is an offshoot of the JAZZ incident thread - but DON'T GO THERE, PLEASE.)

[SAR_YQQ]

The only SCDA approaches that I have flown have been generated by the FMS and flown either Level 2 or 4. You fly the approach down to MDA (+20 or 40') and treat it like an ILS at that point. You either commit to landing or GA. Nice and stabilized all the way down, just need to remember to pull off the power to slow down to Vref.

[Scuba_Steve]

You pretty much have it. The MDA becomes a DH, the idea being that if you fly the SCDA properly then you will hit the DH at the proper Flight Path Angle to execute a stabilized approach.

So if you hit the DH and have no contact you are required to execute a missed even though it will quite often before the MAP.(no turns till after the MAP)

I believe there is also a provision to allow for a momentary dip below the DH/MDA. provided it is for the purposes of performing a missed from the DH.

Cheers

[WayDownTown]

The point of and SCDA is to make a step down approach a constant rate of decent so that you reach the MDA at a point that would allow you to continue at the same rate to the threshold of the runway. It minimizes the amount of time that you are flying down at lower levels because you don’t level off at all. It really isn’t as precise as an ILS as you do a lot of round up of values and you have no vertical guidance other than your calculated descent rate/angle. But the benefit is the stable decent rate which has proven beneficial to the successful completion of an approach or a go around. If you read any Boeing case studies or Flight Safety Foundation articles stable is where it’s at. In order to complete this approach you need to have a few basic tables to determine your descent angle, descent rate and at what altitude you need to start your descent from when you cross the FAF, also have to be mindful the winds and it’s affect on your groundspeed. All you are doing is taking your slope right to the threshold so the further back your FAF, the higher you begin the procedure. If you want to picture it on the CAP draw a 3 degree slope on the profile portion of the chart from the threshold. Where that 3 degree line crosses over the FAF, that’s where you would start the approach and where the 3 degree line cross the MDA is what you would treat as your DH. Once you reach your MDA, now a DH you either continue nice and stable all the way down to the threshold or bug out. There are some limitations to it primarily you need to have a training program in effect but when it works, it works well.

[TopperHarley]

There's no such thing as an SCDA App plate either. You use the regular approach plate and some added charts to calculate the descent point/rate. Its possible to a step-down approach in a transport cat a/c, but the SCDA makes it easier. You also have to add 50ft to MDA to compensate for the "dip below" on a go-around.

[mbav8r]

TopperHarley, You only add 50' if you're doing an SCDA on one engine, otherwise T.C has given a let to dip below 50' assuming you execute the missed at DH. North shore, SCDA is an acronym for Stabilized Constant Descent Angle, as said above you need two charts to calculate the Altitude to cross the FAF and the descent rate from the FAF. The farther the FAF is from the threshold the higher you'll need to be. If we are using a plate that is both an ILS and NDB, we can use the GS crossing alt as our SCDA alt, then at 140kts GS it's about 800fpm, the MDA now becomes a DH. So if you have a strong tailwind and still descend at 800fpm you will be at you DH past your MAP. When I fly them, I'm constantly checking my position and correcting my descent rate to compensate, at times I've had 1100fpm and other times it was looking like I'd be at my DH well back so I decreased the rate to 400fpm.
Generally, if I haved briefed an SCDA but get visual before the FAF, I'll still do it for practice sake.

[Canoehead]

It should be pointed out that it is a "DA" and not a DH or MDA that is used.
Also, 50' can be added for any condition the crew deems necessary (besides the mandatory 50' addition when single engine).

[Rockie]

Read this to learn about SCDA's

http://www.tc.gc.ca/eng/civilaviation/s ... 8-1528.htm

Jeppesen is beginning to modify their conventional non-precision approach charts to depict a DA instead of an MDA, as well as publish a descent angle from the FAF inbound. It's important to realize though that you still must add 50 feet to the MDA or DA unless you work for a company that utilizes the exemption permitting a descent below the DA when conducting a missed approach. Even then there are four conditions that require you to add 50 feet regardless of the exemption.

[turbo-prop]

723.41 Instrument Approach Procedures
(amended 2006/12/01; no previous version)

Stabilized Constant-Descent-Angle (SCDA) Non-Precision Approach

In order to conduct a stabilized constant descent angle (SCDA) non-precision approach, the following requirements shall be met:

(a) the air operator’s flight crew training and qualifications program includes SCDA non-precision approach in accordance with section 703.98 of the Canadian Aviation Regulations;

(b) the air operator’s standard operating procedures incorporate SCDA non-precision approach in accordance with section 703.107 of the Canadian Aviation Regulations, and the procedures include a specified amount to be added to the MDA to compensate for the additional height loss during the missed approach initiation during approaches where

(i) there is a failure of an aircraft system,

(ii) the aircraft is above normal maximum landing weight,

(iii) the aircraft landing weight is limited by aborted landing climb performance, or

(iv) height loss could be expected to be larger than normal;

(c) the final approach course does not differ from the runway centreline direction by more than 15 degrees; and

(d) the descent angle from the planned final approach fix (FAF) crossing altitude to the target touchdown point on the runway is not less than 2.9 degrees and not more than 3.5 degrees.

[dcabrown]

Thanks for sharing the info!

No mudslinging jazz comments, hooo-raaaay!!!!!! I woulda lost money betting on retardo comments making their way in here before the end of page 1...

[xsbank]

I have not seen one of these in North America but the Europeans have been doing these for a couple of years - they have virtually eliminated the "dive and drive" non-precision approach.

When was the last time you conducted a missed approach 'in anger?' I don't mean in training or in the sim. Most of us never get that chance and subsequently a missed approach from an ILS, which only technically allows you to dip 50' feet below the DH, likely will result in a greater loss of altitude. If you are all lined up on the localiser and just can't see anything, then that loss of extra altitude likely won't result in an unsafe maneuver. The danger with the constant-rate approach is that you might not be lined up, you might be past the MAP and you might be descending too rapidly (tail wind? Where have we heard that before?) and your missed approach might result in an unsafe maneuver. You might also be tempted to use that 50' as part of your decision. For that reason, and particularly if the weather is near limits, or if you have not done many of these, or if you are not totally comfortable doing a missed approach (I tell my guys to ALWAYS expect a MAPP and to be pleasantly surprised if you get to land) then I would ALWAYS add 50' to the MDA to make it a DA when conducting a constant rate approach.

Thinking... I don't think you need FMS, I think with raw data you could still conduct a constant rate, but I have only done them with raw data backed up (a requirement in every jurisdiction except Part 91, although still prudent) and actually flown with FMS. My 'raw data' is a VOR approach depicted by the FMS like a LOC with a CDI and (sometimes) a VNAV glideslope.

These approaches are no harder than any other type, what you have to remember is that 'usually' there is a non-precision approach at a particular airport because of terrain or climb restrictions (sometimes its just because they are too cheap to buy and maintain the ILS gear) and therefore you 'may' be descending below MDA/DA which was set where it is precisely for those reasons. Think of an example where the terrain is iffy and ask yourself if you will be safe below MDA in the dark or crap weather. Think of a place you've been where the approach is offset...

Anyway, I guess my long-winded point is: do not descend below MDA until you are completely confident it is a prudent thing to do.

[flyincanuck]

All good posts.

Sometimes even the FMS-generated SCDA will bring your DH "short" of the runway (by 0.5 to 1 mile). I've done a go-around on the SCDA, came around the second time using the the step-down procedure to the MDA and got in.

[55+]

For SCDA...........

The descent gradient for a 3.0 VPA(or GS if you wish) is 318ft/nm.

Multiply that(318ft) by your distance back from the FAF/FAWP and add 50ft for TCH +TDZE altitude . The resulting altitude is what you need to intercept the 3.0deg VPA at the FAF/FAWP. Divide your apch speed by 60 and mulitply that by 318ft and that will give you your rate of descent to maintain the 3.0 VPA.

That's the essence of SCDA and if you fly this on target you will touch down 957ft from the rwy threshold.

have fun with it, ladies!!!!!!

[bcflyer]

We do SCDA approaches on the EMB and must add the 50 to the MDA.

[Airtids]

You only add 50' if you're doing an SCDA on one engine, otherwise T.C has given a let to dip below 50' assuming you execute the missed at DH.

Do you have a reference for this? I'm looking at the PPT TC put together, and it also seems to indicate an "allowed" dip, but I can't find any reference for any of this beyond 723.41.

Thanks

[ahramin]

I think 723.41 makes it clear, unless

(i) there is a failure of an aircraft system,

(ii) the aircraft is above normal maximum landing weight,

(iii) the aircraft landing weight is limited by aborted landing climb performance, or

(iv) height loss could be expected to be larger than normal;

(c) the final approach course does not differ from the runway centreline direction by more than 15 degrees; and

(d) the descent angle from the planned final approach fix (FAF) crossing altitude to the target touchdown point on the runway is not less than 2.9 degrees and not more than 3.5 degrees.

there is no requirement to add anything to the MDA. I do remember seeing a TC AIC which explained that the benefit of a stabilized approach outweigh the downside to descending below MDA:shock: but I haven't come across it lately. It's an embarrassment but pretty much par for the course with TC. Any manufacturer that has thought about it and any ops manual I have ever seen or written requires sufficient height to be added to prevent a descent below MDA, almost always 50'.

You might want to check Advisory Circular (AC) 700-028 Issue 01 – Vertical Path Control on Non-Precision Approaches, dated 2013-04-22.

[BE20 Driver]

These approaches are an attempt to better stabilize an aircraft on approach. There are no level offs, drastic power or configuration changes etc. SCDA approaches are like a non-precision ILS if you want to think of it. You can take any NPA and make it an SCDA by calculating your descent rate using your entry height and your MDA. If there are several intermediate step downs, you must make sure that your descent angle does not dip below any published altitude along the approach. If you are flying an aircraft with an FMS and VNAV approach capability, it will show all of your crossing altitudes in the flight plan. Once you reach your MDA, you must go around if visual conditions do not exist. SCDA approaches are limited from 2 to 3.5 degree approach angles. If you draw it out and fly a perfect 3 degree angle, you will hit your DA further back than a MAP which would typically be over the runway threshold. The charted missed approach assumes that you initiate the missed from the MAP so you are therefore required to remain on the lateral portion of the approach to the original MAP before making any turns.

There is a variation of these approaches, a CDA which basically uses the same process but still maintains the concept of an MDA. Once you reach the MDA, you continue, level until the MAP. Essentially, you are eliminating the other step-down altitudes in a NPA but maintaining the level off at minimums. These approaches are not limited to 3.5 degree approach angles but are limited, at least for Universal FMS equipped aircraft, limited to 4 degrees which is the upper limit of a VNAV computed by the FMS.

In Canada, you are permitted to dip below minimums on an SCDA approach under "normal" circumstances. This does not condone going below, it simply accounts for inertia. If anything is out of the ordinary (engine failure, over landing weight, or if you are doing an SCDA in America, you must add 50 feet to minimums to account for the possibility of a dip below.

FMS are not required to do these as all the information is on a normal approach plate. Jepps include the SCDA angle and a depiction of the DA. I haven't used CAPS for a while but I don't think they have the information printed but it is just simple math to figure it all out.

We have some pretty pictures in our company material but unfortunately, I can not cut and paste it into here.

[Aeros]

You only add 50' if you're doing an SCDA on one engine, otherwise T.C has given a let to dip below 50' assuming you execute the missed at DH.

Do you have a reference for this? I'm looking at the PPT TC put together, and it also seems to indicate an "allowed" dip, but I can't find any reference for any of this beyond 723.41.

Thanks

I realize that this won't help those doing things privately, but for the holders of an OC there is an exemption to 602.128 (2) (b) that allows the momentary descent below MDA even though the required visual reference isn't obtained. There is a requirement to add extra altitude above the MDA in the case of system failures of larger than normal height loss. There is no requirement to apply an additive for a "normal" approach as long as the criteria outlined in the exemption are met.

Exemption

[flyinhigh]

The new CAPS that started to come out earlier this year now incorporate the distance/vertical speed required charts to conduct the SCDA. Jepp has had it for awhile. These types of approaches are the way to go, makes things WAAAAY easier.

One note I will had that I have not saw yet is, if you GPS receiver that is WAAS/SBAS enabled than the unit itself will give you everything from LNAV+V to LPV, therefore even on a standard NPA you will have vertical guidance.

Cheers

[ahramin]

Has anyone seen anything that REQUIRES temperature compensation automatically done by the fms in order to do a barometric LNAV/VNAV? TSO146 maybe? Or something specific to TSO146 installations in Canada?

[Rockie]

Has anyone seen anything that REQUIRES temperature compensation automatically done by the fms in order to do a barometric LNAV/VNAV? TSO146 maybe? Or something specific to TSO146 installations in Canada?

There isn't one. LNAV/VNAV approaches may be flown without temperature compensating any altitudes except the DA provided the temperature falls within the temperature range cited on the approach plate (TLIM). Below that temperature the LNAV/VNAV may not be flown unless your FMS is equipped with and you utilize a certified temperature compensation function.

Within the TLIM range cited on the approach plate obstacle clearance is guaranteed down to the lowest temperature. Below that temperature clearance is not guaranteed hence the requirement for temperature compensation that raises the approach slope back up to where it should be.

If the temperature is below the TLIM and you do not have temperature compensation, you are not permitted to do the LNAV/VNAV but can still do the LNAV only approach to LNAV minimums.

As mentioned before operators can get an exemption to use the NPA MDA's as a DA provided none of the four conditions exist prohibiting it, but it must be remembered that even if you are doing a SCDA conventional non-precision approach CAR 602.128(2)(b) still applies unless you have and utilize the exemption. That's why operators add some amount to MDA unless they have the exemption. For jet operators that's usually 50 feet.


602.128 (1) No pilot-in-command of an IFR aircraft shall conduct an instrument approach procedure except in accordance with the minima specified in the Canada Air Pilot or the route and approach inventory.

(2) No pilot-in-command of an IFR aircraft shall, unless the required visual reference necessary to continue the approach to land has been established,

(a) in the case of a CAT I or CAT II precision approach, continue the final approach descent below the decision height; or
(b) in the case of a non-precision approach, descend below the minimum descent altitude.

[Canoehead]

SCDA approaches are limited from 2 to 3.5 degree approach angles. If you draw it out and fly a perfect 3 degree angle, you will hit your DA further back than a MAP which would typically be over the runway threshold.

Not positive but I believe the range is actually 2.9-3.5 degrees.