Couple IFR questions...

[DynamicallyUnstable]

Ok, so we have an LDA with an MDA of 380' with DME and a 440' MDA without DME. We get down to just above 380' as we do have DME with our GPS. Then *boom* we lose the GPS. Ok, so assuming we did start our clock at the FAF (LOM) and have a time to "missed" at 70 is 2:55, what do we do? We are below the "no DME" MDA already and have no DME. So lets figure this out.
-Do we ascend to the 440' and complete the approach?
-Do we ascend to the 440' and go missed?
----What if we didn't check our ground speed to have the correct timing to go missed without DME?
----What if we did?
-Do we maintain our altitude and go missed with timing?
-Do we go missed automatically? We have to have a point to go missed so how do we find it and what altitude should we be at?
-What do we do if we DO get visual? 91.175(3)

What do you guys think?




And also...

According to 91.175(h), RVR is 1,600' at 1/4 mile. RVR is 2400' at 1/2 mile and so on with the math not working out.
Why isn't 1,600'+1,600' = 3,200', equal to 1/2 mile?
According to math, assuming that 1,600' RVR is 1/4 mile, then 1 mile should be 6,400' but it is only 5000' as depicted in the FARS.
What gives?

[Oogle]

My thoughts (but not up with the American regs):

Track to the missed approach point on climb and conduct missed approach as one of your navaids has failed (DME) and you are now below your published MDA. Go-around and conduct another approach using the higher minimas.

The navaid has failed.

What happens if you become visual at this lower altitude? If you maintain all the requirements for a visual circling approach (ie. constant sight of ground/water, clear of cloud, etc) AND you maintain your obstacle clearance - go for it!

This is all in a perfect world. In the real world .......

[EESDL]

Before the approach, did you brief what you would do in event of nav aid/approach aid failure? You cannot consider using a map if you haven't briefed it!!!!!

If not - carry out a missed approach, re-brief and try again with the higher minimas.

If you did, then surely you were the sort to brief for every eventuality so no snags, it was covered in the brief:-)

(Tongue now out of cheek)
If you have required visual references why would you want to go and do it all again?

Were you single-pilot? Dual controls? Company SOPs?

[C4]

If you started your stopwatch (as you should have), climb to 440 feet and continue the DME out approach. (Same as ILS if glideslope fails during approach, reverts to localizer only minimums.)Ho hum. yawn yawn.

[Oogle]

I wish for once someone can ask a relatively simple question and get a balanced answer without getting strips torn off them by some pilots full of their own self importance!

Please, please don't turn this around on DUnstable as if it were a de-briefing after an instrument rating check ride.

I think it was a great question to ask and hopefully the young guys venturing into IF flying will learn from. Maybe some of the old salts will learn something.

:ouch:

[MightyGem]

hmmm...depends how much you need to get on the ground I suppose. :eek:

[NickLappos]

Think hard about it. The approach is not a purely legal thing, it is a practical navigation instruction. Ask yourself the question "Can I properly follow the navigation instructions?" If so, you are "legal".

If you defined the step-down point with a valid DME, then you have properly navigated to it, and possibly do not require climbing back up to the higher mins. The one case where you might be in trouble is if the MAP were also defined by DME for that lower altitude. If this is the case, then climb back up. If you can execute the published procedure from 380 step-down onward without needing a DME, press on.

Unless specifically allowed, I believe the GPS does not replace a DME for the approach. Is the approach a GPS/LDA?


The RVR conversions allow you to decode the reported Wx with the approach RVR. The conversions are not precise, and not allow mix and match.

[GLSNightPilot]

I hate to speculate without seeing the approach plate. What is the DME used for? For fixing a stepdown fix, for the MAP, or something else? IMO, this really does make a difference. If it's for finding a stepdown fix, and you're past that, then I see no problem with continuing, unless there is something else involved, which I can't see without the plate. If the DME is required for fixing the MAP, then it's a different story. It's theoretically possible for the DME to be required for finding the missed approach holding point, also.

In short, I don't see a way to provide a good answer without more information. Whatever you did is OK with me, sitting back here.

RVR and visibility in miles are different things, and determined differently. RVR is a machine measurement, taken at the runway, and reported in feet. Visibility is determined either by a human observer or a different machine, in a different location, (not at any runway) and is rounded to a fraction of a mile. Being different, they don't correlate exactly.

[donut king]

Good reply Nick!

In Canada, GPS can legally be used in lieu of DME. Just the final course lateral guidance must be traditional navaids( non-gps approach obviously).

Any fellow Canuck please correct me if I am wrong!

DK

[Flingwing207]

AIM 1-1-20 (f)(6)(a) GPS avionics approved for terminal IFR operations may be used in lieu of ADF or DME.

...but the answer (as others have said) depends on what the DME is used for in the approach procedure.

[212man]

Sounds to me as if the MAP is predicated on DME as the poster talks about starting the clock as if it were an additional 'belt and braces' action (i.e. good practice). Therefore, I would say you need to climb up to the no dme MDA. (The loss of G/S on an ILS is a good analogy)

[EESDL]

Oogle
No strip(e)s being torn off just more info being gleaned. Not familiar with FAA regs so, yes, I for one will learn something from this thread.

Making IF approaches in unstable aircraft is a risky business when all is working with you......so yes, brief every conceivable option if you're not planning to 'go around' if one type of approach criteria become extinct.

[inthegreen]

I posted an answer to this question when D.U. posted it on ...gasp...Just Helicopters. Topics go rather quickly there, often off the page before they are really hashed out properly. I like the returning to front and center format here.

Anyway, my answer was close to Nick's, which actually makes me feel quite good about myself.

I'll summarize my thoughts here:

The approach was an LDA, but did not specify DME in the heading, therefore DME is not necessary to execute the approach. As stated, GPS can be substituted for DME in terminal procedures provided its an approved GPS with RAIM and etc. So use of the GPS in lieu was approved.

DynamicallyUnstable posed the question that the GPS(DME) failed after identifying the stepdown fix. Once the fix is identified, the altitude is yours. The fix could just as easily have been identified with a VOR radial, which you obviously could have only identified once. In this case the approach used a DME fix, but the effect is the same. After all, the obstacle that you cleared before the fix is behind you. The terps requirements from this point on are still met.

He stated that he started his time. I assumed this to mean that there were alternate ways of identifying the MAP. One would have been with DME and the other with timing. The AIM does not favor one way of identifying the MAP over another. Obviously, DME can be a much more reliable identification of the MAP in space, because it does not rely on steady airspeed control, although legally, they are equal. As long as the approach plate shows a timing box, timing is allowed. You cannot make up your own timing from distance and airspeed calculations, though.

Legally, you can remain at 380' and finish the approach with timing. In practicality, you would have to be confident that your airspeed was reasonably close to desired, however. You would have to notify ATC of the failure and indicate your intentions. It is possible that the MAP can also be identified with radar.

If your airspeed had been all over the place, and you had been relying on the GPS for the MAP, the best action would be to execute the missed approach by initiating a normal climb to the initial missed approach altitude at least, continuing on approach course to the MAP using timing and execute the remainder of the missed approach procedure from there.

[212man]

DME is not the same as timing and approach minima reflect this (have just looked at several approach plates, randomly choosen, to prove this)! Often the plate will specifically state that the approach is not authorised without DME

The timing is based on groundspeed; not airspeed. If you have no DME or GPS how do you know what your groundspeed is?

[NickLappos]

212man,
In the approach we are discussing, the MAP is not tied to a DME, so the timing is an acceptable substitute. Were the DME used to fix the MAP, it would be stopping you from hitting something beyond the approach that timing might not be avoidable with non-DME timing.
With the ancient method of timing approaches, the error the pilot can assume is worked into the cleared airspace. If you ever saw the TERPS approved airspace for such an approach, it is enormous, and allows for quite gross errors of position, based on the possible errors in the groundspeed and the timing.

If one could see the data used to create the approach, we could see why the MDA lowers when the DME fix is had, because there is an obstruction back there that rises enough to have to be sure to get over before the drop from 440 to 380 can be allowed.

There is an interesting method using probabilities of position that helps create the approach lateral boundaries and the altitudes. The approach is assumed to have a 1 in 10,000,000 safety factor in the primary zone, making the rotor blades less safe than your liklihood of hitting an obstruction.

[212man]

Why specify a different minima for DME than with timing (380 vs 440) then?

[NickLappos]

cause you can't make the step-down without DME. In the assumed situation here, the DME was working long enough to spot the step-down, thus 380 is allowed.
Remember, in a non-precision approach, the MDA can be dropped to immediately after the FAF, perhaps before the obstruction. Thus without DME, you can only drop to 440.