What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Timing an Approach

N

N999BT

Well Known Member
Patron
With all the wiz bang gizmos in our panels these days, I went completely through the training to get an instrument ticket, and I can't remember whether this was ever addressed:

When you start timing from the final approach fix to the missed approach point, do you calculate intermediate values for the time based on groundspeed? If so, how do you do it on the fly when all that other stuff is happening?

For example flying into KCOE on the VOR RWY 5 approach my IAS is 120 knots, and the GS on my GPS says 105. The distance fromn FAF to MAP is 5.3 NM. On the gov't plate the times for 120 and 90 knots respectively are 2:39 and 3:32. If I calculate it out with a calculator (which I currently don't carry in the cockpit, I do (5.3/105)*60 = 3.03 minutes or 3:01.

What do you do? I cant imaginge trying to do the math in my head, and I don't want to get a calculator out and start crunching numbers either. I am open to suggestions.
 
You want to use your best ground speed estimate. Do a rough interpolation if needed. You could also create a chart with ground speeds and distances cross-referenced and laminate it to keep with you to give a quicker answer. Underestimate a bit because if you start a missed approach segment a bit early it is better than starting it a bit late. The time provided on the approach charts is a straight ground speed/distance calculation.

That timing chart is only provided if the missed approach point cannot be identified with the navigation equipment required to fly the SIAP. For example, a VOR approach would have it (unless the VOR is the MAP), even if a DME fix is provided at the MAP because DME is not required to fly the approach. A VOR/DME approach would not have a timing table, and no RNAV (GPS) SIAPs will have one. An NDB SIAP usually would have one unless the MAP is the NDB. It gets complicated, but bottom line, if you have DME (or GPS), use the most accurate method to mark the MAP; the timing table is the least accurate.
 
Last edited:
Do it ahead of time

I always do the math ahead of time and put it on potential approaches. As the other guy noted, a card would be helpful. Also you can always use the higher speed values. Hopefully you won't get into too many scenarios where you don't have a visual by the MAP.
 
If the winds lessen or strengthen, your GS can change substantially on an approach, so unless you are going to recalculating it based on that change, I don't see too much value in performing anything other than an mental interpolation (or a card as was mentioned).

In my limited experience I would rather apply my mental cycles to flying the approach (keeping the needles centered) than trying to whip out the calculator. But that's just me.
 
This is a bit like "measure with a micrometer, cut with an axe". In reality, the ground speed will vary a bit as you descend, as even if you hold a constant IAS, the winds will vary with altitude. A few seconds one way or the other isn't going to make any difference.

I just do a rough mental calculation before the approach starts, based on my expected ground speed. During the approach, if I note that the actual ground speed is different from the expected value, I'll do another rough mental calculation and update the timing.
 
Approach timing

That is why it is called a non-precision approach. :rolleyes: Seriously, the timing is obviously based on groundspeed and the approach plate info is a little lacking. If is is an approach that I do frequently, I just add the "in between" numbers in the margin of the plate while sitting comfortably on the ground. Remember that the timing is to the missed approach point, most likely over the end of the runway. Not much help if you see the runway below you while flying at MDA. The really important, and not always published, number is the visual decent point, that is the last point from which you could safely descend and land. The rule of thumb is 3 times the minimums (AGL) divided by 1000 = miles from the runway. So, if your minimums are 600 feet, 600 x 3 = 1800, move the decimal point over and the answer is 1.8 miles. So, in this example, if you don't have the runway at 1.8 miles, it is time to fly the miss and go to plan "B".

John Clark
RV8 N18U "Sunshine"
KSBA
 
Brantel said:
Not sure I agree with this comment....;)
Click to expand...

I am with you. You may stop your descent, but you continue to fly the segment to the MAP. If you turn to start flying too the missed hold before reaching the MAP, you are no longer in protected airspace.
 
Your example is quite easy

In your example, 105kts is just the mean between 90 and 120kts.
So the time is the mean between 2'39" and 3'32" which is approx. 3'05". That's close enough.
Had you used 110kts, which is 1/3 down from 120 in the 90-120kts bracket, you would have found 2"56" for a true value of 2'54" again close enough for me. (I think I would even round the time difference between 120 and 90 to 60 seconds to make the computing easy.)

Another option is to use the following rule at and around 120kts, each knot difference in the speed results in a time difference of 0.5 seconds per minute of flight (to find the time difference at other speeds, find the time to travel one unit of distance, at 120 kts, 0.5 minute for one mile, at 180kts, 1/3 of a minute for each mile, and use this value in seconds to make your corrections) .
So at 110kts, the speed difference is 10kts, so a time difference of 5 seconds per minute. At 120 kts the time is 2'39" so I have to add 13 seconds to the time to have the time at 110kts, this gives 2'52", pretty close again.

I'm wondering if all this is very clear, reminds me of my IFR training.
My excuse is it was in France, land of theoreticians.
 
Last edited:
Re starting a missed approach early..There is no problem starting a missed approach early. You could start it 1 second after the FAF. There's lots of reasons to terminate an approach early and they could happen anytime between the FAF and MAP.

The key is to never TURN early. In other words when you go missed approach you are expected to climb immediately to the MA altitude and turn WHEN DEPICTED.

If you make a practise of turning early you WILL hit something sooner or later.
 
Thanks John,

This was my point exactly, I just did not have the time to detail it in this manner yesterday....Start it early all you want but if you turn early you might die.

JohnC said:
Re starting a missed approach early..There is no problem starting a missed approach early. You could start it 1 second after the FAF. There's lots of reasons to terminate an approach early and they could happen anytime between the FAF and MAP.

The key is to never TURN early. In other words when you go missed approach you are expected to climb immediately to the MA altitude and turn WHEN DEPICTED.

If you make a practise of turning early you WILL hit something sooner or later.
Click to expand...
 
Thanks for your answer

pgroell said:
So at 110kts, the speed difference is 10kts, so a time difference of 5 seconds per minute. At 120 kts the time is 2'39" so I have to add 17 seconds to the time to have the time at 110kts, this gives 2'56", pretty close again.
Click to expand...

I like this method. I could do this in my head before I get to the FAF. Please help to make sure I understand this. When I apply your method it seems to me that at 5 seconds per minute in your example should be 13 seconds instead of 17. I get this by taking 2:39 is about 2.65 minutes, times 5 equals a shade over 13. I'm not trying to nitpick, I just want to understand this because it is a good way to calculate it on the fly without getting out the calculator.
 
My bad

I like this method. I could do this in my head before I get to the FAF. Please help to make sure I understand this. When I apply your method it seems to me that at 5 seconds per minute in your example should be 13 seconds instead of 17. I get this by taking 2:39 is about 2.65 minutes, times 5 equals a shade over 13. I'm not trying to nitpick, I just want to understand this because it is a good way to calculate it on the fly without getting out the calculator.
Click to expand...

You are absolutely right. Must be the hot weather today here (100?F in my workshop today)
Best regards

PS : I edited my original post
 
You must log in or register to reply here.
Back
Top