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Avionics

When Analog HSIs Die: Repair May Be Best

Weres the common scenario: An owner plans to finally make the plunge to a glass PFD as soon as the aircrafts Jurassic-age analog heading system finally quits. Then it does quit and the dream of adding glass to the six-pack shrivels to budget-stressed reality.Even bottom-line pricing for even entry-level flight display retrofits is far more than it will cost to overhaul or even replace the existing 30-year-old mechanical HSI. While its likely not how you want to spend upgrade money, in some cases it does make sense to repair the dinosaur or upgrade to newer-but still analog-system.

Larry Anglisano
Verified

Weres the common scenario: An owner plans to finally make the plunge to a glass PFD as soon as the aircrafts Jurassic-age analog heading system finally quits. Then it does quit and the dream of adding glass to the six-pack shrivels to budget-stressed reality.

Even bottom-line pricing for even entry-level flight display retrofits is far more than it will cost to overhaul or even replace the existing 30-year-old mechanical HSI. While its likely not how you want to spend upgrade money, in some cases it

does make sense to repair the dinosaur or upgrade to newer-but still analog-system.

Two Good HSIs

We think there are two analog HSI systems worthy of keeping, or even installing in the right cirumstances: the Century NSD360A and the Bendix/King KCS55A.

The NSD360A is a non-slaved (which means you need to correct for gyro precession as you would a plain-vanilla DG), vacuum-driven and self-contained model thats found in various flavors. The double-edged sword with these is that the low entry cost comes with limited life between overhauls and a complicated design of finicky belts, gears and meter movements. A complete overhaul of this unit from a top-drawer instrument lab is just shy of $3000.

There’s also a slaved version of the instrument thats driven by a magnetic heading flux sensor and slaving amplifier. Unfortunately, that adds the potential for slaving component failure. Doing a partial repair to these instruments is money wasted. A real overhaul is usually what it takes for another couple of years worth of service.

Bendix/Kings once flagship, all-electric KCS55A system is far more failure resistant. But when the system does fail, get ready for an impressive shop bill. Thats because there are two high-priced components: the KI525A HSI and KG102A remote electric heading gyro. An overhaul or exchange of either one is several grand. Some earlier KG102A gyros arent worthy of exchange or overhaul due to parts obsoletion, so your only choice could be a new or salvaged one.

Larry Anglisano

Editor in Chief Larry Anglisano has been a staple at Aviation Consumer since 1995. An active land, sea and glider pilot, Larry has over 30 years’ experience as an avionics repairman and flight test pilot. He’s the editorial director overseeing sister publications Aviation Safety magazine, IFR magazine and is a regular contributor to KITPLANES magazine with his Avionics Bootcamp column.