Which of the following is essential when entering a controlled airport traffic pattern without a radio?

When entering a controlled airport traffic pattern without a radio, making visual observations of the traffic flow is essential. This practice is critical for ensuring safety and situational awareness. By visually assessing the movements of other aircraft, a pilot can determine the proper timing and sequencing for their own entry into the traffic pattern. This helps avoid potential conflicts or collisions with other aircraft that may not be communicating, allowing the pilot to make informed decisions based on the observed traffic dynamics.

While starting the approach from the downwind leg and evaluating altitude contributions are important in various flight scenarios, they don't directly address the need for real-time situational awareness in the absence of radio communication. Notifying ground control before entering is impractical without a radio, as the intent would not be communicated effectively. Therefore, focusing on visual observations becomes the most vital practice to safely navigate traffic without the aid of radio transmissions.