There is not much information on the cockpit aspect of the Microwave Landing System. Apart from Wikipedia, there's this excellent 15-minute FAA video from 1974 on TRSB (time reference scanning beam) MLS.

Similar to other precision landing systems, lateral and vertical guidance may be displayed on conventional course deviation indicators or incorporated into multipurpose cockpit displays. Range information can also be displayed by conventional DME indicators and also incorporated into multipurpose displays.

How do pilots fly a non-straight-in (curved path) MLS approach on a Course Deviation Indicator?

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(Source) CDI instrument.

I'll offer an analogy for the simple way I understand MLS: If VOR/DME were to be magically very accurate and to have slant information, an airplane would have been able to perform accurate area navigation using a single VOR/DME station. Instead, this station is the MLS system which covers the approach segment.

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(Source–PDF) MLS depiction from 1978.

With the ever changing track and glide-path, I understand even in the 70's an FMS-like system was installed for the preset approach waypoints—in the lateral and vertical.

Even when coupled to the Flight Director, is there a requirement for a multi-crew, i.e., the Pilot Not Flying would be calling out the upcoming changes? Is there a requirement for an operative (must be on) autopilot? From the FAA video starting at 11:55, I get the feeling that for anything but a straight-in approach an autopilot is a requirement.

It is still being used in Heathrow,* perhaps there's information based on an MLS-certified civilian plane?

Apologies for the mess above, it all boils down to:

From the aspect of instruments and procedures, how do pilots fly an MLS approach?

* The link is now dead, it seems Heathrow no longer offers MLS approaches, but I can't confirm it.


1 Answer 1


Despite all the 'grand scheme' described in the documents you found and the fact that it is all possible, MLS development ran into a few development issues along the way. The biggest was that it was extremely expensive to build for both the ground systems and the airborne receivers. That slowed its development as its benefits over ILS weren't justified by the costs.

One big benefit that your reference describes is that it actually allowed for area navigation. Where ILS just gives you a deviation from a path, MLS gives you a 3-D position (Azimuth-Elevation-Range) that can allow for the RNAV paths. But this was complicated and then the second big shift arrived -- GPS. Since GPS gives you RNAV capability globally, why spend a fortune on something that would work only locally with very high infrastructure costs? Based on this, the FAA pretty much abandoned MLS in the late 80's in favor of expanding GPS/GBAS/SBAS systems.

Europe kept on with MLS work because they had a different issue with multipath interference to the ILS systems. The effort though just focused on the final approach segment. GPS/RNAV systems can handle the inital portions of the approach just fine. To keep things simple, and to make them retrofittable into existing aircraft at a reasonable cost, the MLS receivers were designed to provide an "ILS Look-alike" output. And to avoid dealing with new procedures and training, they kept the ground systems and approach design limited to ILS-Like functionality -- just with higher precision and less interference issues.

The US does have some MLS approaches, but they're mostly 'private' approaches paid for by the airlines that want them. The FAA won't fund 'Standard MLS approaches'.

As a result, current MLS equipped aircraft can fly MLS approaches that are operationally identical to an ILS CAT III approach. Most aircraft use a multi-mode receiver (MMR) that allows for both ILS and MLS approaches (and some now also support GLS). The difference is that instead of an ILS frequency, the pilot selects an MLS channel. Approach procedures (and the approach plates) are basically the same as an ILS, so crew training is minimal.

So the answer is that today, you fly an MLS approach just like an ILS approach.

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    $\begingroup$ @ymb1 Yeah, right. Compared to ILS, you just move from an AM radio with a static antenna array to microwave phased array scanning beams with DPSK coded data words interspersed in the stream. To be fair though, the costs did drop in the 90's with the introduction of custom ASICs and FPGAs. But today, it's still at 3 times as expensive as ILS. $\endgroup$
    – Gerry
    Mar 16, 2017 at 12:30
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    $\begingroup$ @ymb1 Thanks for catching the typo in the acronym soup. $\endgroup$
    – Gerry
    Mar 16, 2017 at 13:15
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    $\begingroup$ @Gerry: TRSB-MLS was adopted in 1978 by ICAO (UK tried to impose the Doppler-MLS). The first GPS satellite was launched the same year and GPS was operational (24 satellites) only in 1995, still military (US), and subject to selective availability until 2000. So I guess at the time MLS was a good strategy, the only credible alternative to ILS and the best choice for curved approach. $\endgroup$
    – mins
    Mar 16, 2017 at 17:38
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    $\begingroup$ @mins It was a rational decision at the time. GPS was the disrupter technology. The strange part is that after the FAA moved towards GPS, the USAF actually moved to add MLS to its capabilities as it solved an issue with all-weather ops in bare sites. The key advantage was the minimal setup time. They went out in the late 90's to create mobile MLS (MMLS) ground stations that could be deployed in a C-130. Most tanker and transports are now MLS equipped. $\endgroup$
    – Gerry
    Mar 17, 2017 at 11:51

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