Starting 172s With Alternator On Vs Off

Starting 172s: Alternator On vs. Off – Which is Better?

This article explores the age-old debate among Cessna 172 pilots: should you start your engine with the alternator switch on or off? Understanding the nuances of this seemingly simple question can significantly impact your engine's longevity and overall aircraft operation. We'll delve into the pros and cons of each method, helping you make an informed decision based on best practices and safety considerations.

The Core Question: Why the Debate?

The debate centers around the potential strain on the battery and starter motor. Some argue that starting with the alternator on places an unnecessary load on the system, potentially leading to premature wear and tear. Others believe that leaving it on aids in a quicker and more efficient start. Ultimately, the best approach depends on several factors, including your aircraft's specific condition and the prevailing environmental conditions.

Starting with the Alternator ON: The Pros and Cons

Potential Advantages:

• Faster Starts: In some cases, having the alternator engaged during startup might provide a slight boost to the electrical system, potentially resulting in a quicker engine crank. This is more noticeable in colder temperatures or with a slightly weaker battery.
• Reduced Battery Strain (in theory): The argument here is that the alternator contributes to the electrical load during cranking, thus reducing the strain solely on the battery. However, this is debated heavily as the alternator may not immediately generate enough power to make a significant difference, and may instead increase the load on the battery.

Potential Disadvantages:

• Increased Load on the Starter: The alternator adds to the overall electrical load during the starting process. This increased load might put extra stress on the starter motor, potentially leading to faster wear or even failure in the long run.
• Potential for Backfeeding: In rare instances, a malfunctioning alternator might backfeed power, potentially damaging the battery or other electrical components. This scenario is more likely with older or poorly maintained aircraft.

Starting with the Alternator OFF: The Pros and Cons

Potential Advantages:

• Reduced Strain on the Starter Motor: By turning the alternator off, you isolate the electrical load to the starter motor and battery, potentially reducing stress on these critical components. This is generally considered the safer and more conservative approach.
• Minimizes Risk of Backfeeding: With the alternator switched off, the risk of backfeeding during startup is virtually eliminated.

Potential Disadvantages:

• Potentially Slower Starts (Marginal): Without the alternator's assistance (which is often negligible during the initial cranking phase), the start might take a fraction of a second longer. This difference is generally insignificant in most circumstances.
• Higher Battery Strain (if prolonged cranking is needed): This will become apparent primarily if you experience issues like a weak battery or very cold weather.

The Verdict: Best Practices for Starting a Cessna 172

While the debate continues, the generally accepted best practice is to start your Cessna 172 with the alternator switch OFF. This approach minimizes the risk of placing unnecessary strain on the starter motor and the battery, and avoids the potential complications of backfeeding.

This conservative approach aligns with principles of preventative maintenance and extends the lifespan of critical components. The slight time difference in starting the engine is generally inconsequential compared to the potential long-term benefits of reduced wear and tear.

Remember to always consult your aircraft's Pilot Operating Handbook (POH) for specific recommendations and procedures related to starting your engine. Your POH is the definitive guide for your aircraft's operation and maintenance. Regular maintenance, including battery and alternator checks, is also crucial for maintaining optimal performance and preventing issues.