RTA as an arrival manager tool

[themwasthe days]

I am undertaking some research into the use of RTA in the current fleet in Europe, and how best it can be incorporated into a ground based arrival management system.

Basically, what is being considered, is to combine the use of this onboard system functionality (in all its variants and updates) where fitted to a FMS, into a ground based trajectory prediction system that will take the responsibility for streaming arriving aircraft within a TMA so as to reduce pilot and controller workload, improve predictability of trajectory (speed and altitude) and thereby increasing the ability to fly CDA techniques and provide a higher adherence to P-RNAV routes where published (minimise radar vectoring). Pie in the sky? Possibly, but with better processing and improved harmonisation of pilot SOP, maybe it can be done.

So, to get things going;

• what levels of accuracy can be expected;
• what issues need to be considered with respect to applying additional constraints where there are existing constraints already hard wired into the system (STAR);
• how effective is the use of RTA at a downstream waypoint in predicting a take-off window from a departure point say within 30 minutes of the RTA waypoint;
• what strategies are/can be applied to meet RTA

There are many reports written on how this functionality can be utilised but little in the way of hard data as to how successful it has been in the real world. The myriad of issues surrounding FMC performance variations is something we need to get a handle on, however, the impact that may have specifically on RTA maybe limited. What I would like to hear are experiences from those who have had the opportunity to use it in the real world, and what the issues are.

Thanks.

[Golden Parrot]

I have played around with the RTA function of the FMC on the various 'busses I have flown and personally find it very poor for several reasons:

First of all for the system to have any chance of accurately predicting an arrival time it needs the proper route programmed into it and followed. So shortcuts, wx diversions, altitude changes etc. all combine to reduce accuracy. Also the forecast winds need to be accurately inserted for all applicable levels AND they need to be similar to the actual winds...not always the case.
Now whenever any of these parameters change the systems responds by varying the cruise speed. Several hours out it might only be by Mach .01 but the closer you get to the time restricted point I've seen speed fluctuate from just below MMO down to Green Dot (minimum speed). This obviously causes potential separation loss with other traffic if several aircraft are flying hugely varying speeds on the same level/route.

[arba]

They do it in Singapore , after giving standard arrival, ATC gives you required
time arrival over certain waypoint.

cheers

[Old Smokey]

They do do it in Singapore, and always for a waypoint on descent - ANNOYINGLY!

The problem is that RTA works very well for a waypoint in cruise, but NOT for a descent waypoint, VNAV PATH is in sole control in this phase. (At least for Boeing aircraft, I cannot speak for Airbus).

Also in Singapore, it appears to be beyond ATC's capabilities to advise of a RETA when 45 minutes or so from destination when you could do something about it. I think that Singapore ATC must have shares in the sub-contractor building speed brakes.:uhoh:

Regards,

Old Smokey

[Blip]

I actually think the RTA feature on the B737 works really really well!

I know that is not the opinion of the vast majority who try it. I agree it CAN really muck you around and cause all sorts of issues on descent such as choosing a descent speed lower than that required at the waypoint in question and/or changing the descent speed midway during the descent which can have a profound effect on the descent path (steeper/shallower). However if you keep a number of points in mind, you can consistanty make good an RTA to within 15-20 seconds (yes even those ones on descent) and if required loose around 5-7 minutes per 100 nm.

The trick is to:

1. Go to the LEGS page and enter the IAS that ATC want you to maintain from the RTA waypoint.

2. Go to the PERF LIMITS page and enter a limit on the MIN descent speed. The lower limit of IAS on descent should be equal to the speed ATC require you to maintain from the RTA waypoint (usually 250 kt). If the FMC should choose a higher descent speed than that for descent, that is fine as slowing down prior to arriving at a waypoint is handled very well by the FMC. It's when it has to speed up prior to a waypoint that it gets into trouble. i.e if you want to maintain 250 kt IAS from the chosen waypoint, and the descent speed chosen by the FMC prior to arriving at that waypoint is 265 kt, the descent profile will be fine.

However if the FMC chooses a descent speed of 220 kt to make good the arrival time at that waypoint, the descent path from that waypoint onwards will still be based on a descent speed of 220 kt!

Remember a speed entered in the LEGS page is actually an AT or BELOW speed. The FMC has chosen a speed BELOW 250 kt. Once you have passed the waypoint you would then have to enter 250 kt in the descent page and execute. This might put you high or low on the new descent path and you may have to put on thrust to accelerate and/or regain the new descent path. (I have seen descent speeds lower than 250 kt cause a descent path to be steeper than that built on a 250 kt descent.)

The fact that you have put a lower limit on the DESCENT speed forces the FMC to calculate a lower CRUISE speed than it would have otherwise chosen to make up for the higher descent speed. Remember too that it will NEVER choose a speed (Mach) less than that which equates to the best Holding Speed (IAS).

3.If the minimum speed (best holding speed) will not be slow enough to make good the required time of arrival, a CDU scratch pad message "UNABLE RTA" will appear. However this is only true for the current cruise altitude entered in the CRZ page. Remember for a given IAS your TAS decreases with decreasing altitude! With one CDU on the RTA page, the other goes to the CRZ page and enters a lower cruise altitude. Notice the effect this has on the waypoint ETA. It will be getting closer. Keep entering a lower and lower cruise altitude until the FMC indicates it can now make the required arrival time. You might have been cruising at F350 and the required level might be F300 or F250. You then simply have to request descent to that level.

4. If descent is required, do so in (Flight) Level Change or equivalent so that the descent is made using idle thrust and the TAS reduces as quickly as is possible. Make sure the MCP target speed is IAS (not Mach) and that the IAS is set to the best holding speed (found on the HOLDING page).

Remember too the effect of wind at the new level. You might be descending out of a jet stream. A decreasing tailwind will have a profound effect on the resulting ground speed. Descending out of a headwind will obviously have the opposite affect and may cancel the reduction in TAS.

Here are some numbers to keep in mind.

IAS @ FL = MN TAS 100 nm in: Time lost

254 380 (.80) 462 13.0 min -
(230) 380 .73 425 14.1 min 0.9 min
(230) 300 .62 367 16.3 min 3.3 min
(220) 250 .54 325 18.5 min 5.5 min
(220) 200 .49 300 19.9 min 6.9 min


Perhaps in busy airspace you may not have the flexibility to descend as required. Also you may not want to descend as there may be weather below you or reports of clear air turbulence.

Not sure if this answers the questions askeds but I thought it might be useful anyway.

Cheers.

[Rainboe]

This was experimented with in BA on the 737 LHR fleet about 10 years. Very limited usefulness. You had to try and get airborne at roughly the right time. We all know how successive approaches and a take-off queue can seriously mess that up. Then really there is a fairly limited range of speed available to the crew to control Fix Arrival time. So large variations in cruise/descent spped, combined with manpower planning and issuing RTAs- why not just let aeroplanes get away when they can and ATC controls them inflight and handles them during descent as needed than plan for flights that may well not make RTAs? It's something that looks great in planning that doesn't stand the test of the real world.

[themwasthe days]

Blip,

The info provided is of great value, and I thank you sincerely.

In getting a better understanding of cost benefit in using RTA to absorb some of the delay, (in the cruise mode) can you complete the table below by giving a summary of the fuel used (B738?) for each segment, assuming level flight for each segment?

IAS FL Mach TAS 100 nm in: Time lost Fuel req’d

254 380 .80 462 13.0 min
230 380 .73 425 14.1 min 0.9 min
230 300 .62 367 16.3 min 3.3 min
220 250 .54 325 18.5 min 5.5 min
220 200 .49 300 19.9 min 6.9 min

A similar study undertaken a little while back, using a A310 as the subject, and a 200nm sector requiring an ATC constraint of 45 minutes, demonstrated a saving of approx 600kg when compared with a “standard” procedure of maintaining F390 for 30 min and 15 minutes in the stack at F050 as compared to descending to F250 and min speed for 40 min and 5 min in the stack. The issue is of course, to have in place some form of “contract” that guarantees the slot in the queue when aircraft have conformed with the RTA requirement issued by ATC.

WRT the issue of departure slots, would not the departure window offered by the FMS provide some greater flexibility (for both pilot and ATC) than the current CFMU slot? Assuming a flight of 60 minutes from airborne to RTA WPT (say 120nm from destination), what ‘window’ could be expected?