TOD technique, and how it is calculated?

[LikeABoss]

Just wondering how the TOD is calculated?
I see on the ND a blue arrow (I think it was blue) which represents where the descent should start.
Why does the MCDU chose that, based on what exactly?
If we did not have that indication, how do we calculate when we should start our descent?

Descending down from FL300, following the STAR and being given radar vectors and getting cleared to intercept the LOC at 10 miles and 2000 feet AGL, how does one get himself to be sorted out so he doesn't have to increase rate of descent, use spoilers as I have been reading that can also be hectic with the VLS speed dropping low, etc.

Just a rookie looking to be prepared :)
Thanks all!

[BOAC]

Flt Sim enquiry perchance? If so, I recommend
Spectators Balcony (Spotters Corner) If you're not a professional pilot but want to discuss issues about the job, this is the best place to loiter. You won't be moved on by 'security' and there'll be plenty of experts to answer any questions.

[bingofuel]

Rule of thumb, from FL300, divide by 30 and that is number of track miles for the descent, so start 100 miles out. Close enough for government work!

[john_tullamarine]

The calculation looks at the distance needed for the aircraft to descend in the air mass at the weight and ambients existing.

Operational data (FCOM and similar - which is built into the black box) will present suitable information for the pilot to find this.

Looking at still wind conditions

(a) a typical jet profile is flown generally at reduced thrust initially and then flight idle. This typically approximates something in the vicinity of 3nm/1000ft with a few miles to slow up and reconfigure for the approach. To this must be added/subtracted a few miles to allow for wind during the descent (which one may or may not have with any accuracy)

(b) turboprops may be a bit more flexible and will vary somewhat more according to Type

(c) unpressurised aircraft will be limited by cabin ROD so, typically have a much flatter descent path depending on speed.


The black box, depending on the accuracy of the data fed to it may or may not do a good job of figuring the descent.

The pilot, with knowledge of his/her Type and an imperfect knowledge of ambients, likewise, can make a pretty good/not so good estimate on the day.

Knowing the approximate profile required, the attentive pilot can modulate the ROD/IAS slightly on the way down to effect small changes to accommodate variations on the day. With a bit of skill and attention, the typical descent can be initiated and thrust not need to be played with until spinning up for approach.

So, for instance, years ago on the Electra, we had a short sector (HBA-LST) which we often flew with no other aircraft about. Generally was a dollar on the sector to see how close the pilot could go to making TOC=TOD with an aim then not to touch the throttles until rolling out on final. Great fun and made for a sharp mind .. all hand flown, raw data, of course.

ATC, on the other hand, can impose short term tracking requirements which totally throw the descent planning to the winds and, in such cases, the pilot needs to be able to replan on the fly. Often this will require out of sequence and normal flow reconfiguration to change the basic descent path for the aircraft.

For a large aircraft, one can play with a descent from

(a) however shallow is required by keeping thrust on, through to

(b) the typical 3nm/1000ft plus/minus a bit, through to

(c) 1-1.5nm/1000ft if the need arises.

Most jet Types have a problem with pressurisation with steep descents so that often will constrain flexibility for passenger operations.

If circumstances conspire on the day to put you ridiculously high, an orbit or two, or track stretching is used to lose height.

In summary, the black box is only an aid to doing it .. the pilot still is (should be) the entity driving the show.


The previous post is a simple way of using the basic calculation for nil wind

FL300 = 30,000ft x 3 /1000 plus a bit = 90 + 10 = 100 nm which is a pretty typical jet starting point.

[fantom]

Or, you could just go down on the arrow.

[john_tullamarine]

Or, you could just go down on the arrow

I don't know how good the current lot are but, in days of yore, that was a guarantee of the wrong point to start down.:E

.. noting, of course, that the typical Phantom driver had a little more flexibility that his Boeing and Airbus cousins ...

[Love_joy]

Flightsimmer or not, you're more than welcome to come in here and ask questions - so welcome. It's only when armchair pilots start telling us how it is that we'd have a right to get on the defensive.

I'm on my second aircraft type, and both types operate along very similar lines.

To keep it simple, a typical rate of decent that works well for everyone in your commercial jet, is approx 2000ft/min. So, if you were cruising at 36,000, you would need 18 mins to hit sea level.

Using 360kts ground speed, that works out at 6 miles a minute. So for every 2000ft you drop, your covering 6 miles. 6 x 18 = 108nm (very close to the 100 mile rule of thumb above).

I use this exact technique above, and it serves me very well.

You have to bear in mind however, that as you descend the wind varies, and most airspace in the UK requires you to be 250kts or less below FL100. So you have to slow up somewhere too.

Also, which the tree huggers are still reworking the airspace to enable Constant Descent Approaches (CDA), many STARS require you to descent early and fly level during the arrival (France and Germany are especially bad for this).

With regards to the FMS, they all do more or less the same thing. Most allow you to specify heights for the waypoints as required, and allow you specify the rate of descent or desired glidepath between the points. Using vertical nav on the flight director will then vary the decent as require to hit the altitudes at waypoints as required.

Lastly, to fly a 3º decent profile (ILS, NDB, VOR APPR), the rule of thumb is 5*groundspeed. For 100kts, thats 5*100=500ft/min.

[Intruder]

At normal cruise speeds, initial ROD from the mid- to high-30s is closer to 3000 FPM, and decreases to 2000 FPM around 10,000'. The 300 ft/NM gouge is better, because the glide ratio will be relatively constant.

[CAT3C AUTOLAND]

The 3 times table works wonders here, which is of course based on a 3 degree slope, 318 ft per nautical mile.

3 times your height above the ground, for example, FL390 would required 117 miles, plus the speed you need lose, typically descending at 300 knots, you need 1 mile per 10 knots, so coming back to clean speed, will need about 10 miles. Also take into consideration the wind, usually a mile per 10kts of wind adjusting for a head or tail wind.

All this stuff will come with experience, and you learn quickly what your aircraft is capable of.

[FlightPathOBN]

Back to the question...as JT stated, the TOD is calculated based on a number of real-time and coded data...the same aircraft, under different loading, atmospheric conditions, and other variable, will calc a different TOD.

In this example, the same aircraft was used and recorded....

http://operationsbasednavigation.com/wordpress/wp-content/uploads/2010/11/ToDvariationsV1.jpg

[FlightPathOBN]

yes, all to same airfield, same approach, etc...

[FlightPathOBN]

DJ,

This was a study done for RNP procedures. The FL340 level then a waypoint and speed restriction of 250 at FL100.
This study was to look at the variations of the FMS calculated TOD with the same aircraft.