When to start the APU at 30,000 feet after an IDG BUS EICAS message on a SkyWest ERJ

Outline at a glance

• Understand what an IDG BUS EICAS message signals

• How the SkyWest ERJ powers it all: IDGs, EICAS, and the APU

• The altitude rule you’llWant to remember: 30,000 feet

• Why this matters for Cockpit Qualification (CQ) and Knowledge Validation (KV) topics

• Quick, practical takeaways you can apply in real flight and in study conversations

Now, let’s walk through it in a way that sticks.

What that IDG BUS EICAS message really means

Picture this: you’re cruising along, the cabin is comfortable, autopilot is doing its thing, and suddenly a caution light or EICAS message pops up—IDG BUS. That message isn’t just a blip on the screen. It signals that one of the Integrated Drive Generators (IDGs) isn’t feeding power to the bus system like it should. The IDGs are the workhorses that generate electricity for essential systems in flight. When one goes quiet, you’ve got less electrical redundancy and less margin for the bridge between engines and electrical power.

If you’ve ever played with a power strip and you unplug one core device, you notice the others have to pick up the slack. In the airplane, the stakes are higher. The flight deck relies on that electrical backbone for flight guidance, lighting, avionics, navigation data, and emergency systems. So when IDG power is down, you don’t just lose a gadget or two—you tighten the margin on how you’ll keep the airplane safe and controllable.

A quick tour of SkyWest ERJ’s power layout (in plain terms)

• IDGs: Two units typically, each capable of powering essential buses. If one IDG goes offline, you’re down to the other, which can still support the airplane—but it’s a tighter balance.

• EICAS: The electronic checklist that flags anomalies in the cockpit. When you see IDG BUS, it’s a heads-up that a generator isn’t delivering as expected.

• APU: The Auxiliary Power Unit acts as a back-up generator and a source of stable electrical power when the engines aren’t providing enough. It’s the “backup plan” you want online when a generator is compromised.

• Electrical load and essential buses: The airplane divides power into essential and nonessential paths. In a partial power loss, the priority is keeping flight-critical systems alive and healthy.

So what do you do if that IDG BUS message appears while you’re cruising?

Here’s where the altitude rule comes in, and it’s more about timing than panic. The guidance is anchored in maintaining adequate power with redundancy as flight conditions change. If you see the IDG BUS message, you plan for a reliable power source to take the load if the remaining IDG can’t cover it all, and you time it so you’re protected during the most taxing stretches of the flight profile.

The altitude threshold you need to know: 30,000 feet

The specific rule you’ll hear in cockpit discussions and in CQ/KV topics is this: the APU should be started when you’re below 30,000 feet if an IDG BUS issue occurs at cruise. In plain English, that means as you descend through 30,000 feet, you should have the APU up and running to restore and maintain electrical power to essential systems.

Why that exact number? It’s tied to how engine-driven electrical power scales with altitude and engine performance. At higher altitudes, engines run more efficiently in a different way, and loads on the electrical system can shift. The threshold is set to ensure you don’t push critical systems to their limit at a time when electrical generation is at a higher risk of lag or dropout. In practice, you’re aiming to keep essential systems alive with a stable power source, and the APU provides that fallback when you slip below the 30,000-foot line.

Let me explain it with a quick mental model. Think of the electrical system like a city grid. The IDGs are major power plants; if one plant goes offline, you don’t want streetlights and hospital power to flicker because the backup plant (the APU) hasn’t fired up yet. The 30,000-foot rule is the city’s prudent moment to flip the switch and ensure that power remains steady as you descend toward routines like approach, landing, and the option to divert if needed. It’s not about a drill or a checkbox exercise; it’s about keeping safety margins intact when the air gets thinner and the generator count dwindles.

What this means for Cockpit Qualification (CQ) and Knowledge Validation (KV) topics

If you’re studying CQ and KV, you’re likely weighing how pilots think through system faults under pressure. This scenario is a textbook example of ensuring power redundancy and orderly restoration of systems. Here are a few practical takeaways you can fold into your mental model:

• Prioritize power sources: When a generator warning appears, your first job is to confirm the available power sources and their loads. The APU is not just a backup; it’s a deliberate tool to preserve essential systems while you troubleshoot.

• Understand the thresholds: The 30,000-foot mark isn’t arbitrary. It reflects a balance between engine performance, altitude, and the electrical load that the aircraft can safely sustain with one IDG out of the loop. Memorize the altitude cue and rehearse it in your mind along with other power-related procedures.

• Coordinate with the crew: These scenarios aren’t solo missions. You’ll work with the rest of the cockpit crew to confirm APU start, verify electrical power, and manage flight path to a safe altitude if you need to minimize risk. Communication, as always, is part of the plan.

• Tie it back to systems knowledge: The value of this rule lies in how well you connect it to the bigger picture—how the IDGs, the EICAS messaging, the APU, and the bus architecture all interact. CQ and KV aren’t just about memorizing a number; they’re about understanding why that number exists and how it protects the flight.

A few practical notes to anchor the idea

• When you’re above 30,000 feet and IDG BUS appears, you’re analyzing more than just one warning. You’re validating that the system still has viable alternatives and you’re preparing to sustain essential operations as the airplane changes its power balance.

• As you descend below 30,000 feet, you’re in a better position to bring the APU online in a controlled manner, while still preserving flexibility for any needed deviations or contingency planning.

• In real life, procedures are layered. The APU may come online, but you’ll also check that other generators and essential buses are powering the right circuits. It’s about a methodical restoration, not a rush to push buttons.

A quick, memorable way to approach this in discussions or on exams

• “IDG BUS warning? Start the APU by the time you cross the 30,000-ft line.”

• Visualize the “power ladder”: engine-driven power (IDGs) at the top, APU as the backup rung, and a final safety net in the form of proper load management and feeder buses.

• Think of the 30k threshold as a safety net for altitude-related power margin. It’s not just about a moment on the flight deck; it signals a broader approach to maintaining system reliability under changing flight conditions.

Relatable analogies that land the point

• It’s like driving with a spare tire in reach. If you encounter a tire problem, you want the spare ready before you head into rough terrain, not after you’ve already worn the tread down while climbing a hill.

• It’s also like managing a home’s electrical system during a storm. If one line goes out, you flip on the backup generator so the essentials—refrigeration, heat, lighting—stay on while you figure things out. The cockpit version of that mindset is exactly what this rule embodies.

A few longer, more formal notes for clarity (without getting too heavy)

• The IDG is a generator powered by the engine. If it drops, the aircraft must rely on alternate means for power. The APU provides that alternate source. The rule of 30,000 feet is about ensuring you’re not left with a fragile power state during critical phases of flight.

• EICAS messages are not just warnings; they’re prompts for a stepwise, safe response. Recognizing the context and the right altitude cue helps you transition smoothly from alert to action.

• The broader CQ/KV learning objective here is to develop a mental framework for how electrical power is managed in flight. Awareness of how and when to bring the APU online reinforces good decision-making under pressure.

A closing thought that ties it together

Power management is a quiet, steady art in the cockpit. It’s less about dramatic moves and more about timely, well-considered actions. The IDG BUS warning at cruise isn’t just a moment of stress; it’s a chance to demonstrate how you apply system knowledge to preserve safety and stability. Remembering the 30,000-foot rule gives you a reliable compass to navigate through that moment with confidence.

If you’ve ever watched a flight deck documentary or chatted with seasoned pilots about electrical systems, you’ll notice one common thread: the best explanations are grounded in real-world reasoning, not memorized lines. The 30,000-foot threshold is a crisp, practical guide that helps pilots keep the airplane safe while they assess and address the fault. And for learners exploring CQ and Knowledge Validation topics, it’s a perfect example of turning theory into reliable, actionable cockpit behavior.

In the end, you’re not just studying a rule—you’re building a mental toolkit. A toolkit that helps you stay calm, stay informed, and stay in control, even when the air gets a little unpredictable. That’s the kind of clarity that makes sense of the SkyWest ERJ’s electrical heartbeat—and the kind of clarity that makes you a better pilot, one decision at a time.