Why do I not always hear ATC read back by pilots (on required instructions)?

Question

I enjoy tracking air traffic at my local KORD. I listen on LiveATC and use my private virtual radar setup to get "real-time" traffic info.

I understand which instructions need to be read back by the pilots per this question however on more than one occasion I don't hear read back on critical vector info on departure, despite the visual confirmation of instruction (pilot making proper vector and speed adjustments).

I tend to notice this with bigger birds (777,747,340), however smaller regional jets almost always promptly read back.

Questions:

1. Is there an alternative way of ATC instruction acknowledgement? (Other than read back?)

2. Is it possible that the reply is somehow on a different frequency?

3. Is this just a problem with LiveATC? (One theory is that A/C leaves receiver coverage area and that's why I don't hear reply, however on approach side much bigger distances are heard in my area)

Thank you

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I did verify that indeed the aircraft that I don't hear read back from receives special departure frequency by tower.

Once tower clears aircraft for take off and hands them off to departure controller they use "departure" frequency to read back instructions. It is typically different from standard departure frequency.

The only thing that is not clear is if there is any rhyme or reason for which aircraft gets this special departure frequency and which doesn't.

As pointed out in the question for some reason mostly it is "big" aircraft that gets this preferential treatment, but I am not 100% sure why.

Answer 1

The O'Hare Eight Departure is the only departure procedure out of ORD. This is what it looks like:

Notice that there are three departure control frequencies:

• Chicago Dep Con West: 125.4
• Chicago Dep Con East/North: 125.0
• Chicago Dep Con South: 126.635

Airplanes are usually given the frequency appropriate for the sector that their first departure fix is located in. If they are very busy, each sector will have its own departure controller and they will be talking to different people. If it isn't as busy, they pilots will still be talking on their individual frequencies, but one controller may be listening to and transmitting on two or all three frequencies at once.

For instance, if a particular aircraft is cleared over POLO VOR (due west of ORD) they will be given a departure frequency of 125.4 as part of their clearance. Let's say that the traffic is slow and only one controller is being used to cover all three frequencies. If you are listening to the East/North frequency on 125.0, you will hear the departure controller issue a clearance to the aircraft, but never hear the response because the aircraft is transmitting on 125.4.

Answer 2

At my airport, KCOS, tower has split frequencies for the east and west runways. 90 percent of the time both are handled by the same controller. They transmit and receive on both frequencies, but you are only listening to one.